As for seperate forms, I'd expect that. Seeing as they've given the Notch Eared Pichu its own OW sprite, and the Pikachu-Colored Pichu its own OW sprite. Then, I'd expect other forms of Pokémon to be shown in their own OW sprite, such as the Rotom forms and all. However, as for a Shiny OW sprite following us, that is questionable as they may have only done it for the Pikachu-Colored Pichu, but you never know.
Ok but what if you where able to see the pokemon you where floating on? They would probably just mak a sprite for the pokemons that can learn surf but what if someone with an action replay and pokesav make a giratina lear surf or something else would they make a sprite for that?
Lol everyone would be smushed by the Wailord... I'd probably take an Heracross or a Granbull.hmm, I'd have Giratina follow me, for a normal Pokemon....maybe Skarmory..
OH, IMAGINE A WAILORD following you o_O
I think it's a little unnecessary, but I suppose it'll be interesting to get to see whatever Pokemon you want following you. I hope you can choose though, since sometimes Pikachu in Yellow got a little annoying, and it'd be nice to not HAVE to have something following you if you want.
I'm pretty sure it's optional. In many of the pre-release screens you don't have any Pokémon following you; so unless those were taken before the feature was implemented, you shouldn't have to worry about disabling that feature.