If you must have Balthier with guns (i know i do) then Machinist/White mage is pretty good.
I have the same setup as you, aditionally:
Fran: Archer/Red Mage - Fran has to run bows with me for lore reasons, luckily Red mage gets Ardor, a sick flame spell that is boosted by The Burning Bow. Aditionally Fran has White magic 1 naturally, which helps Red Mage since it doesn't learn cure.
Ashe: Knight/Time mage - Ashe has excellent stats for all roles, but higher magic growth. She'll be a physical powerhouse and gain access to higher level white magic later due to Knight. Her high magic will help getting time magick off too. It also fixes the hole i create by making Balthier a healer, since she'll support him well
Basch: Bushi/Uhlan: Basch has bad magic and vitality (vit is good for a breaker) and high combo speed with katanas. Uhlan will give him more versatility and even more strengh.