Okay, any FF movie would probably be very bad, very bad, but here goes...
Terra:
Locke: Stephen Baldwin
Mog:
Edgar: Heath Ledger
Sabin: Scott Caan
Shadow: Mark Dacascos
Celes:
Cyan: Viggo Mortensen
Gau:
Setzer: Benicio Del Toro or Colin Farrell
Strago: Ian McKellen
Relm:
Kefka:
Umaro: Tyler Mane

Haha, well damn, that was hard as hell...little help with the blanks?