Sorry for the late reply! I've been so busy lately X.x
Hmmm these days I'm into RNGing in game found pokemon - I find it fun somehow :D
Also, I find that by the time I get my RNG'd pokemon, the usage of that pokemon has dropped Dx
I've never taken my shaymin into sky form...considering the fact that Skymin was uber and I stay away from that tier...but I think it still can. Just because the event was hacked doesn't make the pokemon hacked in any way. You still save in front of the pokemon and do the coinflips and walk and paralysis/sleep false swipe to get it lol.
Some people will not allow for hacked pokemon to be used in online battle until they are somehow released in a legitimate way (Shaymin was released via wifi in a cherished ball for instance but Keldeo wasn't released yet so you can't hack an event and use it).