To Battle a Gardevoir would be unsettling and cheap. Its psychic, correct? so it would easily know who i chose/send out, and know what moves i would choose.
Not much different to you knowing what Pokemon your opponent is going to send out next, to be honest. =p
I am not sure about it, tbh. After all, it doesn't fit in any of the canon unless you look as Mewtwo in the movies, I suppose, and that's the sole example I know of (and certainly not a game-specific example either - so if Game Freak haven't introduced that 6 gens in it'd really seem odd for it to suddenly be a thing. Sure, you could say the same about Fairy type, but... I just don't think it'd happen, nor does it appeal to me.