Yeah, I did a playthrough of 7 with forced classes on each character.

Cloud - Time Mage (also with Restore for Regen; I made the Cure spells out of bounds)
Barret - Knight (with Counter as well)
Tifa - White Mage (her ultimate weapon sucks so she may as well be one)
Aeris - As long as she lasted, I made her a Sage and gave her the best Black and White spells available at the time
Red XIII - Black Mage (he suits it; starts off with Fire)
Yuffie - Ninja/Thief (Steal, Throw, 2x-Cut etc)
Cait Sith - Blue Mage, but next time might give him ailments and leave him in Fury state so he can be a Gambler a lot
Vincent - Summoner. Anything else would be made useless by his Limit Break.
Cid - Oracle/Green Mage


This adds a lot more fun as you'll sometimes switch different characters in for certain situations. Until you reach the end and you only use Cloud because you have to, Tifa for her White Magic and Yuffie for 2x-Cut. I guess Red, Cid and Vincent were my least used here, though Vincent was fun to have in some boss fights; he'd do most of the damage, then when he ran out of summons he'd morph. Awesome!