All starters will probably get some sort of mega evolution, but I don't think the balance will be a problem, since you will only be able to have one mega evolution per battle & I believe they will be banned from tournaments since they will be as strong as legendaries. And on top of that, the mega stones will not be able to be obtained through gameplay, but by other means such as downloads etc.
That's not what I heard from this:
With the official Japanese site update, a small note came out regarding further clarification on the Mega Evolutions. Within this site, it details that with Mega Evolutions, you can only have one Mega Evolution Pokémon in battle. It's not known whether it's talking about all of the battle, or just within your team. This is presumably to allow for a specific balance in the Pokémon. It also clarifies that the Pokémon will revert after the battle has ended, not within it itself when the battle finishes. We're still not 100% on the specifics of this new process, but we'll keep you updated as more information comes
Kalos Region Champions X and Y
Mega StonesMega Evolution is not possible for all Pokémon. Only certain Pokémon, under certain conditions, can achieve Mega Evolution. One of those conditions is a special item called a Mega Stone. These special stones exist for every Pokémon that can Mega Evolve, and each is named for the Pokémon to which it grants power.
To use a Mega Stone, give it to your Pokémon to hold. Pokémon capable of Mega Evolution will gain access to their potential only if they first carry a Mega Stone into battle. Other things are needed for Mega Evolution to succeed, and those secrets will be explained later!
Some of the many Mega Stones seem to be hidden in secret places in the Kalos region. Finding every one of them will be one of the challenges awaiting you in Pokémon X and Pokémon Y!