I'm pretty sure you do not choose the order the characters enter battle, but rather the order their abilities activate in. They all enter battle at the same time, so any character ability that affects a character in battle can do so to a character who has been banded in by SS, even if they have not yet activated their ability.
As for order, I'm pretty sure you get to choose the order of activation as a whole; that is, you can choose to activate Gabriel, then Michael, then Haman, then Captain of the Host, if you really want.