the guns on a fighter plane/starship

I think with some WWII fighters, the pilot could change the convergence point of the forward guns. Guns could be set to fire straight ahead in a spread pattern, or they could gimble them slightly to set them to coverage at some point in front of them for concentrated firepower. IIRC, the X-Wing from Star Wars had the same user selectable convergence point as well. It was a topic in the Rogue Squadron books. Wedge Antilles preferred straight firing guns while other pilots preferred them to converge.

With a tank, they are crewed with 2-3 people on average so one person can man and aim the MG, while someone else aims the main gun. So I don't know how much that works with this discussion.

Not in flight as far as I'm aware. Hell, even the P-86 Sabre couldn't do that.

Guns were fixed in place and the convergence point had to be set by the ground crew during maintenance manually. The actual point was laid down in official regulations, which in 1940 was 200 yards for RAF fighters officially although some pilots managed to convince the ground crews to shorten that to around 70 yards.



For modern tanks the typical crew number is 4; Tank Commander, gunner, driver and loader (depending on if the design includes an autoloader) Historically tanks have had crews ranging from 1 (The Stridsvagn or S-Tank can be operated and fought with only a single person) to two dozen (Some of the German A7V tanks from WW1)

WW2 tanks usually had crews from 2 (some French designs designed post WW1) to eight (Some of the M3 Grant series) although a more typical figure was 5 (Commander, gunner, loader, driver and radio operator/bow gunner)

In practice it is not a great idea to have three man crews unless you have an autoloader and two man crews are utterly impractical even if you do. The TC is meant to be looking for targets/threats and working the radio so anything that takes him away from those duties puts the tank at risk. The Gunner is sitting in front of the TC and doesn't have a huge amount of room, with the ammunition being stored on the other side of the turret to where he is sitting. So while it is possible for him to act as both gunner and load this drastically lowers his ability to locate targets and rate of fire. And the Driver is not physically in the turret and depending on the design might not easily be able to get from the hull to the turret to help out even if totally losing your mobility isn't a concern. (It's also not a great idea to try to get from the hull to the turret when the latter is traversing as it tends to result in losing body parts)

Modern tanks don't, of course, have a bow gunner with the machine guns being operated by the gunner, TC and sometimes the loader might have a gun to play with as well. It is also true that modern tanks may on occasion also end up as a three man crew with the TC taking over the loaders duties due to illness or injury. (Fyi; Its also true that in some tanks the TC has controls that allow him to aim and fire the main gun, or in the case of the T-55 is seated so close to the gunner can simply lean forward and operate the gunners controls)

The French had some 2 man tanks during the mid war period, which also saw action during the Battle of France. These types of design, even when armed with only a machine gun, were not that effective due to the commander being overloaded with duties.

Another factor not being considered for tank crews, but which might also apply to spacecraft, is that three people make for a more stable crew than two; one will act as a mediator in cases of disputes.

The British and German's had similar designs, the Matilda I and the Panzer I both of which were two man tanks armed with machine guns.

The Matilda I saw action during the Battle of France, and indeed a counter attack from Matilda I's almost killed Rommel and staggered the German advance long enough to allow the British (and some French units*) to withdraw to Dunkirk. The design however ended production in mid 1940 and was withdrawn from front line service. Although they would remain as guard vehicles at airfields in both British use and the Germans would likewise use captured Matilda I's as guard vehicles; A standard practice as they would use captured equipment both in first and second line units throughout the war.

The Panzer I would be the principle tank for the German's, in terms of numbers during the Battle of France and remained in front line use until 1942. With a limited number of redesigned recon versions seeing action in Normandy in 1944. The design would even remain in service until the mid 1950's in Spain.

Two man tanks all seem to come from the mid war period, which was a time of experimentation with the new technology to see what worked and also a time of low military spending. Two man tanks were, in this regard, tempting as they could be smaller, cheaper and built in larger numbers than three or four man tanks. It is also interesting to note that those two man tanks that saw service during WW2 were a result of swift military build ups, where there simply wasn't time or money to build up numbers of larger and more capable designs.
We need to be a little careful when talking about such designs, especially when looking at Japanese and Italian tanks, as our opinions of their abilities involves comparing them to later generations of vehicles; Japanese tanks are a good example of this. In 1943 they were no match for a Sherman. But at the time they entered service in the mid 1930's they were actually just as good, if indeed not better than, the principle tanks being used elsewhere at that time.

(*People keep forgetting that Dunkirk wasn't just the BEF but also French troops. They also tend to forget that the only reason the BEF was able to withdraw was due to the heroic actions of a number of French units which held the German's off allowing the last of the forces to embark on the ships.)

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In regards a space craft (or other fictional combat vehicle) the desirable number of crew might depend on not only the combat workload for the crew. But also how long it is likely to be in the field and away from a base - keeping in mind that even in a war actual combat tends to comprise a small percentage of operations for the majority. This could actually be an interesting notion for a space-fighter depending on the level of the technology.
If you are talking about, say, the technology of the Star Wars or Star Trek universes then 'fighters' are likely to operate in a similar manner to modern fighter craft in which they would, at most, be only a few hours away from a base in which repairs and maintenance can be managed by 'ground' crews. In this case flight operations would be short and the crew focused on piloting their craft without concerns regarding security or maintaining their vehicle. As such smaller crews are viable and numbers (and combat effectiveness) would be based on combat workload and nothing else.
If however you are talking about more 'realistic' technologies them even 'short' ranged fighters would most likely end up spending several days to several weeks away from their base while on patrol. In this case larger crews become more practical, since the crew is likely to have to deal with their security (ie one person keeping watch while the others sleep) as well as conduct repairs and maintenance away from base. In this regards a larger crew is useful as it lowers the individual workload outside combat. (This is also one argument against using autoloaders on tanks and moving to a three man crew)

The Italian 'Tankette' was based on a British design that was in turn designed and produced by the same company that would go on to make the Matilda I and the Universal Carrier (The most produced armored combat vehicle in history).

The British design also acted as a basis for the Panzer I and many other similar light tanks and Tankettes from the mid 1930's.

These designs are what could be considered 'pure' Infantry support vehicles rather than tanks. Being in effect a mobile tracked machine gun emplacement that lacks the ability to deal with fortifications. Compared to a 'true' tank which is basically a form of mobile field artillery. (By this logic neither the Matilda I or Panzer I is an actual Tank, where as the Matilda II and Panzer II are)

It's worth keeping in mind that, as has been pointed out by many others, if you have a 'tank' and your opponent doesn't then your tank is by default the best one on the battlefield. This seems to be the logic behind the 'Tankette', as they were largely developed and used by colonial powers in areas where the opposition didn't have anything to counter them. This is largely the case for the Italian Tankette's, and indeed Italian tanks in general. Although the situation there is more complex as Italy was attempting a massive military expansion and modernization with a limited industrial base with which to do so.

The L3/33 used a 43HP engine and weighed just over two and a half tons.
The Carden Loyd tankette from which the L3 was derived had a 23HP engine and weighed one and a half tons.
The Matilda I used a 70HP engine and weighed eleven tons
The Panzer I used a 59HP engine and weighed just under five and a half tons.

Interestingly the fastest of these was actually the Carden Loyd with a top speed of 30mph, and the slowest the Matilda who's top speed was a massive 8mph. The L3 and PnZ I 26 and 23 mph respectively.

But then this does seem to fit the roles for which they were intended and principally used.
The Carden Loyd and L3 were principally intended for recon, the Panzer I more as a training vehicle although it went on to become a recon and command vehicle. The Matilda a heavily armored infantry support vehicle.

In WWII, the US had technological superiority, but it was focused back home on the manufacturing end, not on the front line weapons, at least for most of the war. The idea was, while the Sherman tank was not the most bleeding edge tech thing on the battle feild, it was fast, maneuverable, easily easily repaired, and very reliable, and could be manufactured enmass. Also easier to ship overseas. The US out produced German tanks by about 4:1

The US eventually did start building tanks on part with the German ones, but they were late in the war and only fielded in limited numbers. The M26 Pershing didn't hit Europe until 1945.

By the end of the war the B-29 was one of the most advanced planes built at the time and it was built enmass.

One of Germany's most common failings is that they pushed too much on designing new stuff that it took resources away from supporting and improving what was already in production.

There are a lot of myths about American and German technology during WW2, especially tanks.
The Sherman was, on its introduction in 1942, probably the best tank in the world. It's gun, the short 75mm, was at least comparable to everything else being used except the German 88mm gun on the (very rare) Tiger's*. The only problem with the gun was with the ammunition, which was partly solved by mating captured German warheads onto American cases and then in fairly short order by better American AP rounds being produced**
The armor was only slightly thinner on the front than the fabled Tiger I due to the Sherman's being sloped.
Sherman's blowing up when hit was due to the ammunition not the fuel, and was a problem other Allied tanks had at that time.
The 'five Shermans to take out one Tiger/Panther' is only true in so far that when told there was German armor on the field and the infantry needed armored support. The response was to send the smallest tactical armored unit available. This, in the 1940's, was five tanks.

(*Total production of both Tiger versions was around 2500, almost all of which ended up on the Eastern front. In North Africa the Afrika Corps had a grand total of 18 Tiger I's.)
(**The original American AP round had a tendency to shatter when it hit armor at under 500 yards. Even when a better round was available, which was fairly quickly, American Tankers preferred the hybrid German-American round. This was due to the German AP rounds having a bursting charge which American rounds lacked.)

Also note; The wonderful, 'much better than the Sherman' Pershing tank was withdrawn from service in the US Army in 1951. The Sherman remained in service until 1957 and is still in front line use in some parts of the world.

But don't just take my word for it. Here are two videos by someone who knows what he is talking about;

Watch the first video;

US Army doctrine for tanks specified that part of their duties was to deal with other tanks.
The Tank Destroyer branch was for defensive operations, as specified by their doctrine.
The TD branch specified that tanks should fight other tanks.
The 75mm gun on the Sherman was specifically chosen because it was good at anti-tank work. Not only that but it was the same gun as was carried on many of the Tank Destroyers.
The majority of German tank units used the Panzer IV, which is directly comparable to the Sherman in terms of weight. Once the ammunition problems were sorted out the Sherman had no problems with the Panzer IV.
There were only three recorded tank vs tank battles against Tigers by American armor. In the first the Pershing lost. The second the Sherman's won. The last wasn't a fair fight as the Tigers were being loaded onto trains.