-snip-
Their level doesn't matter nearly as much as how many stats are going up each level. Just get them roughly as strong as the rest of the team, or at least good enough that they won't die if something sneezes at them. If you try to level them up and they get few or no stats each time, drop them in favor of someone else.
Mia's kinda redundant because you already have Ike as a swordsman, but you should probably get Soren a bit stronger if you can. He can potentially attack 4 times per combat (Adept yo), and if his stats go up properly he has a not-insignificant chance to crit on every one of them. He's a solid mage and does an excellent job fighting other mages IIRC.
Not that Mia's bad or anything - I prefer her to the other myrmidon you get - and once she hits Swordmaster she can do an excellent job holding chokepoints all on her own.