You kinda work with both then.
With you switching in and out Party members, you are free to either keep the main character either the same class, or switch it around a bit, because either way, you're going to be looking for a good class combo.
And with that, you can go for a slight FF approach, (Out of the games I've played of their's anyway...) Where, you keep the same character, but let them throughout the game decide on the right class.