File:Tennin (Japanese angel).jpg - Wikipedia, the free encyclopedia Pokemon that already can fly without wings. We have categorised them as 1) Eastern Dragons that can float due to mystical powers. (like Gyarados). 2) psychic powers of levitation like Mew. 3) Legendary Pokemon that is just that powerful that they can float. (like Landorus) 4) Rocket engines. just to be more broad ability to use fly(Genesect) http://bulbapedia.bulbagarden.net/wiki/File:649Genesect_BW_anime_3.png) There is no other categories.
Now to place Sylveon in one of this category it would have to be 2) psychic powers of levitation like Mew. 3) Legendary Pokemon that is just that powerful that they can float. Finally by making thus 2 & 3 much more common they would have succeeded in opening pandora's box. Thus enable new type of flying pokemon that do not visually look able to fly.
Girly type pokemon. Honestly its girly. I would not mind to have a boyish pokemon that would then share the same type Sylveon.
About multitype I think this eeveelution is special a duel type would however be defeating the idea behind eeveelution.