My current setup:

Vaan: Thief
Daggers and Ninja Swords with Light Armor.

Balthier: Alternate Thief
Poles and Guns with Light Armor.

Fran: Archer/Freya
Bows and Spears with Light Armor.

Basch: Warrior
Swords and Hammers/Axes with Heavy Armor.

Ashe: Alternate Mage
Katanas with Light Armor.

Penelo: Mage
Rods and Maces with Mystic Armor.

I switch between two parties: Vaan, Freya, Ashe, and Balthier, Basch, and Penelo. Everyone is always withing 2 levels of each other.