No, none of the sliver commanders or non-legendary slivers are banned in Commander
In Mega Gardevoir's case, all of its pre-evolved forms, including regular Gardevoir, are present in the background, creating a family portrait-style scene
In the Commander Bracket system, game changers are only unrestricted at the higher level brackets, with only three allowed in the third bracket and none in the first two
I would try and keep this card on your bench until you get four energy cards on it, then put it into the active Pokemon spot and use Psychic Protection every turn, you should be able to last quite awhile in the active Pokemon spot without getting knocked out and be able to knock out quite a few Pokemon before having to retreat
Most of the unique mechanics the main selling point of the play experience are indifferent to the format, so long as it's Two-Headed Giant.
Final Thoughts Mega Evolution has officially established its place in the Pokmon TCG spotlight despite just releasing