I recently bought a KAC SR25 ECC off of a buddy of mine.  It came with the KAC 762 MAMS muzzle device.  From what my friend told me I can only use either the MAMS or QDC flash hider, as they are specifically designed to create enough back pressure to eliminate any short stroke problems that might occur with a 16" 7.62 barrel with a rifle length gas system.  Is this true, or can I throw a SureFire SOCOM 762 Muzzle Brake on it?

First off, congrats on your purchase.  I use the MAMS on my ECC but I have friends who use AAC brakes on their ECC and EMC so I would assume Surefires would work as well.  A muzzle device doesn't affect the gas system by the way since the bullet passes the gas block before exiting the muzzle device.

Your friend has misinformed you. However, most KAC enthusiasts, including myself, regard the MAMS as the best. Let me know if you remove it and want to sell it via PM.

I second this. MAMS > SF brake any day. I literally shot them back to back on the same rifle, and the MAMS did a better job at controlling muzzle movement with less side concussion.