I’m probably late in posting this but:
I don’t play Fire Emblem, I don’t really care much for it, nor do I know too much about it aside from a few things here and there, but when it comes to representation in Smash, having just Marth (with Lucina and maybe Chrom as skins akin to Alph and the Koopalings), Ike and Robin, and probably a villain is the best way to go about it.
Marth, Ike and Robin represent 3 different eras of FE, have unique playstyles (light and quick swordfighter, heavy and powerful swordfighter, and mage swordfighter), and are all popular in their own right. Lucina (and Chrom kind of to an extent) can easily be just a skin for Marth if not kept as an echo fighter.
Roy is only around in Smash still, mostly because of fan demand resulting from Melee nostalgia (he and Marth were the first taste of Fire Emblem for the West) and how he was made more unique from Marth after the latter got a number of changes in Brawl.
Corrin, iirc, was put in mostly as a promotion for Fates, despite having a unique moveset, and could easily be swapped for the protagonists of later FE games as a “flavor of the month” deal.
But I’m probably just talking out of my own ***, since I’m not very knowledgeable on FE, and because I believe that 7 characters is way too many for FE in Smash.