They can be dual-typed, but at least one of those has to match the type you choose. What type are you choosing, and what’s your team look like?
I would have to choose Bug-type, of course. Team composition would be Galvantula, Yanmega, Heracross, Centiskorch, Genesect, and Rabsca. As to why… I just like 'em, lol. I’m not much of a competitive player. What would your team/type be?
I’ve never had the chance to use Sylveon, but I’d love to! I’m a pretty big fan of Eevee and all its evolutions. Also, of all the types to build a team around “design aesthetic” Fairy is probably the most appropriate, lol!