Yes to the second, no on the first.
In gas guns, as you increase BB weight your muzzle velocity goes down but muzzle energy actually creeps up and despite the urban myth it's not the velocity that really matters the most. Sites always set their FPS levels "when chrono'd with a 0.2g BB" for that very reason, they're actually checking your muzzle energy but nobody ever phrases it that way.
Energy = Mass x Velocity squared, and all that jazz.
Could you elaborate? I've never ever heard of a magazine that was incapable of holding a certain weight of BB. Actual ammunition can feature numerous weights of bullet, but the dimensions of the round (and therefore the capacity of the relevant magazine) stay exactly the same, that applies to airsoft too.
Woops. my bad :L I didnt mean the magazine holding the weight of the BB, i mean (And i'm reading this from the manual) that the gun, or to my knowledge, the barrel can only shoot 2 types of rounds between 0.20 and .25 gram BB's, nothing heavier is referenced, so im assuming this is the norm for all GBB guns?
Also, I haven't necessarily changed out my rounds from 0.20's, so im unaware of any first-hand experience with heavier BB's and sometimes passed the thought that it the higher the gram, the bigger the BB