versatility,
a stack of 10/10/10 tac/stk/int planes can be used to attack inf. armor and defend if necessary.
what is not clear:
if the target unit consists of 8 inf and 2 lt. tanks, does the battle calculation use 8 tac and 2 atk planes?

Yes, planes have the same 10-unit maximum when calculating contributions to damage. Whoever told you to make a 4/3/3 tac/AB/int stack was wrong; that sounds like a terrible stack that would be mediocre at everything and good at nothing.

It's better to design stacks for specific purposes. Tacs are much stronger in early game than ABs because of the prevalence of unarmored units, so you can just make a stack of 10 tacs if you're looking for some extra firepower. Going beyond 10 units will be situation-dependent: for example, adding ints to the stack will help them defend against enemy ints, but if your enemy is not using ints then why bother?

The high speed and maneuverability of planes also means you can mix and match stacks for situations much easier than land units. For this reason, I rarely stack above 10 land-attacking planes unless I'm padding HP. With land units, you want a well-balanced stack because you may not know what type of units you're going to come up against. With planes, you decide when and what to attack, so just optimize for the specific stack you're attacking. If it's mostly unarmored, throw 10 tacs at it. If it's mostly armored, throw 10 ABs at it. If your enemy has ints, add ints of your own or send your ints ahead to scout and patrol.

And just remember that in wars against capable, active players, ints will decide air superiority. In those case, either commit to air superiority and make sure you have a lot of ints, or ignore planes altogether.

Another technique is thinking of how to sequence attacks against certain stacks. If the enemy has a mixed stack of unarmored and armored (for example, the current meta-stack of regular AA + RRG), you should consider where most of the air defense is coming from and consider taking that out first. In our AA+RRG example, despite the scary units being heavy armor and thus targets for ABs, you might want to hit it with tacs first to weaken the AAs, saving damage from your ABs.

Yup, correct.

Possibly, yes, depending on research level for you vs your opponent. But if you go too heavy with destroyers, they're then vulnerable to naval bombers...

No, the damage potential is calculated separately for each armor class. In your example, your 10 best planes for light armor would be used to calculate the damage potential against light armor and your 10 best planes for unarmored would be used to calculate the damage potential against unarmored.

More general to the question, ignoring the expediency of time, division of labor etc., it is always equally or more efficient to have stacks as large as possible. For example, say you are bombing a target that takes more than 1 strike to finish, you will lose less HP if you attack with 11 TACs instead of 10. I would suggest playing around with the battle calculator to get a feel for these less obvious interactions.

I'm not a proponent of only using 10-stacks and not overstack. Some reasons for overstacking an air stack:

- Adding an interceptor to a 10-bomber stack for stealth spotting;
- Mixing tacs and AB's for doing better damage against mixed ground stacks ("best ten" againt unarmored are different that "best ten" against armor, see DxC's post). When I build AB's at all, I often group them with tacs in 7/7 stacks or something like that;
- Adding int's for better protection, OR for attacking an air base with a ground stack in it;
- Mixing NB's and ints when attacking a carrier group.

...and that's just from the top of my head. As a rule of thumb, grouping units in larger stacks will take longer to achieve the same (bombing) result, but they will have a better "damage output to losses taken" ratio.

All of that is situational though; don't think in terms of "ideal stack", but merge and split your stacks as the battlefield situation develops.

In general it helps to overstack units that cost less resources/time per HP to absorb damage. As an example, naval bombers are significantly cheaper than TACs in terms of resources/time per HP. If you have a bunch of naval bombers sitting around and they have the range to the target add them to your stack and let them take the damage. I made a little table that shows the resource cost per HP for units at different levels: Resources per HP Table

Can you tell us something more on how to read that? For example, why was food excluded?

oops, just forgot food. I fixed it. It's simply the given resource amount needed to produce a unit at the given level divided by the HP of the unit at that level. All values are without any doctrine modifications. Time is the minimum time in hours, but since it's for comparing different units/levels it's ok.

wanted to delete this post but iam still failing, sry
(so iam still wrting here stuff: a weitghing option for the different ressources could be an Idea or that we could sort it by armor/unit type)

mp is manpower

Yes, it's pretty common for manpower to start to dwindle after a day or so, which is around when I start to build recruiting centers. MP is also the one resource that is impossible to acquire other than generating your own, since the other resources can come from captured provinces, trade on the market, or trade from other players, so it's usually a little harder to manage.