Is there a quick summary somewhere?
Not that I know of. You can check the
Ufopedia, but that's not short. For the most part, it's intuitive enough.
...asterisk in its name (like PKM / *PKM). What does it mean?
Pre-disable the weapon so your craft won't use it in combat until you re-enable it, either
in combat or clicking that spot again.
Does it matter what machine-gun I'm mounting? Like is there any difference if it's a PKM or a Minimi for example?
Depends. A big minigun is double the DPS of a small one, which is double again of a BO LMG.
OTOH, early craft, yours and the enemy's, are so fragile the combat is a crapshoot anyway.
I see that I can mount a Rocket Launcher on my Humvee. What rockets will it use - small, big, shrapnel? Does it matter?
Small. They cost less?
Can I shoot them down if I mount a Rocket Launcher?
Even the PKM has better DPS than the Rocket Launcher, and there's no range difference. The RL has vastly less ammo, too. So unless you're betting on alpha-striking them with quad rockets (and I think the odds are against enough of the rockets connecting for that to work), use bullet sprayers. I liked miniguns several versions ago.
Chinooks are tougher than your craft, but should be less well armed. So I'd guess you can shoot them down with anything, but there's always the chance it'll go wrong.
In general, early combat craft are either really slow or limited-range. I wouldn't bother unless the enemy tries to mess with your transports.