Barrett= Ving Rhames(MR. T just seems to clownish)
Sqaull= His name won't come to me at the moment, but he played Anakin in Attack of the Clones
Irvine= WWE superstar Rob Van Dam
Seifer= Chris Conrad with blonde hair
Quistis= once again the name won't come to me, but she was Jean in all the X-Men movies.
Zell= Seth Green
Cid Highwind= Keifer Sutherland(It's a longshot, but I think he could pull it off)
Seymour/Vincent/Sephiroth= Johnny Depp(I think he could pull off all three roles)

FYI, I'm pretty sure Porm and Palom were a boy and a girl