What if it is like it is now. If you don't have that skin downloaded, it shows default.
Therefore, lets say there's 100 skins for the Pony. You can only use 15 in game, like now, but you are able to choose those 15. Then, you will be able to fly that skin and see it if another player is flying the same skin. Those who don't have it downloaded only see the default skin.
I'm pretty sure that we choose the skins based on individual preference (someone chooses to fly a skin because they like it.) I think there's really no point, other than variety and squads, in seeing other people's skins.