You're not going to draw more than 15A with any airsoft gun unless something is going on with your motor.
Most stock guns draw anywhere from 4A to 12A, mostly dependent on the motor.
Like a stock systema Magnum may draw as much as 30A because it's arcing through the end bell. But a 400fps 1300rpm gun with a well tuned motor may only draw 15A.
20C with 1200mah is very safe. 20C with 3300mah can handle pretty much anything you hook up to it lol
And yes I do run mosfets on all my guns.
IMO it's pointless to run LiPo without even a simple mosfet, mainly due to safety. Having a mosfet with a resettable fuse may save your lipo from toasting. And most importantly, it will save your trigger contacts, not just from a higher voltage of the 11.1v, but from the higher amp load of a LiPo, which even a 7.4v on sustained fire could cause your contact block to melt.
If you want more info on LiPo, they're explained in great detail here;
http://www.airsoftcanada.com/showthread.php?t=99899