FF1: Typical: Fighter/Fighter/Black Belt/Red Mage
I would probably just add an extra Fighter or Black Belt, so I don't know that it changes how I would play all that much.

FF3: Knight/Dragoon/Ninja/Devout
I would add a spell caster, I tend to run really on the physical side....

FF4: Ditto on Yang.

FF6: Celes/Terra/Locke/(one of the Figaro Brothers)
I would probably add whichever other Figaro Bro which I wasn't using, which is kind of boring.

FF7: Cloud/Cid/Yuffie
I would probably add Red XIII, but mainly for story reasons.

FF9: Zidane/Eiko/Amarant/Freya
I don't care much for the other characters. Early game I would add Steiner, as I don't have to worry about MP, later in the game Vivi with Osmose.

FF12: Ashe/Balthier/Bash
Whoever I add would probably just become another clone, so I don't think it makes a difference.