Q: are the milliamps the ''gas tank''? more mAH and longer flight time?
Yes, the capacity of the battery is an indicator of the amount of available energy to do work. More capacity equals more work.
Q: ok now, can i put a 20C battery in a 30A esc? or does it have to be 15C?
Absolutely. The C-rating is just an indication of how much current the battery can provide. For instance a 2000mAh (2Ah), 10C battery can provide 20,000mA or 20A of current. That doesn't mean you have to pull that kind of current out though; it will be just as happy or happier providing 5A. It is the motor/prop/plane combination that determine that.
Q: can i use a 2100mAH, 20C, 3S in both of these planes, if not what do i need to do to maKE that happen?
Yes, provided it will physically fit into the airplane. Should be no problem in the Corsair. I'm not familiar with the other plane though so I can't really say for sure.
The main reason that the Corsair comes with a 15C battery is twofold: first, it doesn't really need it anything more and second, it keeps the cost of the airplane package down. You can run a 20C, 30C or whatever you want in it though. In fact, I'd never purchase batteries under 20C since the prices are so good now.