I plan to download Y later tonight, also, I plan to get Froakie as a starter, but completely ditch him when I can get charmander.
From my understanding of the game, you can have more than one mega evolution, but just cant activate more than one per battle (someone please correct me if I'm wrong). If this is the case, then I plan to get both Mega Gyarados and Mega Charizard Y on my team.