slight spoilers maybe, not plot specific just about leveling characters and stuffs. so...
SPOILERS
im like 40 something hours in, right after the draklor laboratory. All my characters use sword+shield except for fran who is stuck on bows. They all use swords because i have like 6 Deathbringers, which is a pretty awesome sword you can get kinda early on, so it kind of ruined my class specific characters. I mainly use Ashe, Vaan and penelo, Ashe and penelo are my support characters and vaan is mostly into technics. Also i wouldn't worry about some of your characters being lower leveled early on, when you get to that first town in the empire you can get that item that gives you 2x exp. The embroidered pin.. or something. Vaan was level 45 and basch was 28.. and i leveled him up to 48 in like 4 hours or something like that.