Yes, generally your party will probably become a core unit.. with maybe a couple units swapping in and out to fit specific battles. There's no real point in using all the characters unless you want to grind a lot in sidequest, or want more of a challenge. Of course the one reason to use all the characters are for support convos.
Oh...this seems like this would be kind of hard cause they are tossing units at me left and right. I know Chrom, Sumia, Avatar, Lisse, Sully, and Stahl are my current mainstays right now and I'm trying my damnest to level Donny from 2 to 3 without him dying.
I'm pairing my avatar with Tharja because they're both Dark Mages, but he doesn't like her because she's stalkery :-(
I wonder would my Speed plus do good for their kids together. Originally I was gonna do Magic as my thing but this Speed boost got me 2x enemies left and right. My avatar got the highest speed in the unit.