Every one of the Scions (save Alphinaud and Alisaie) use standard player bodies and animations just with custom faces/hair; with the possible exception in some cases of headgear, they can codewise wear anything a player can. Adding more outfits for everyone but the twins would be trivial; you'd just go 'hey, let them use this existing gear for glamour'.

The twins might take a bit more work, since I'd be shocked if most gear had a variant for the 'Elezen youth' body; players can't use it, so there's no reason for it. (Plus it seems that the twins share a body—along with every other Elezen child you ever encounter. If you examine the game's resources in a model view, I'm fairly sure difference in body shape between the twins is part of Alisaie's outfits, not the actual character model. Which means even if outfits might work on Alphinaud with minor tweaks, Alisaie would probably require custom ones.)