There's only one flaw with that plan. Most likely someone will give you the stone that you Mega Evolve with. Like this, if you choose squirtle, they'll give you the mega stone for squirtle and you can't get the one for charmander. Now how do you get the Charmander Mega Stone for both versions?
Well it's not been proven right or wrong, I do predict a lot of things, but sometimes I do get them wrong, that's just what I belive will hapen, it might not, just wanted to let you know that it's a strong possibility.
Well either way I was part right (meant to comment sooner) but if you havn't already noticed, you can get all the stones. Well if you already have your game, I would love to battle, you can reply if you want to battle or trade.