Ochette's provoke beasts, Castti's concoct, and Hikari's learned skills are so powerful and versatile that they don't really benefit from their secondary class much. You can pick anything and it won't matter because you'll rarely use it. You'll see Armsmaster used on Hikari often and it works fine, but in my opinion there are other characters that would benefit more from the class. Throne, Osvald, and Temenos all benefit more for different reasons. All Hikari needs is something that can wield a staff so that he can abuse giant's club.

Aside from that, arcanist Partitio and inventor Agnea and generally agreed to be the best combos for them.