I don't think anyone has mentioned this yet.. (if someone already has, my apologies!)
Would a Light Screen/Calm Mind set be worth considering? Given Sylveon's support role, you could set up CM to boost it's SpA/D to increase its own utility while putting up Light Screen to aid the rest of your team. Could make for some interesting special wall scenarios, shame it doesn't get Reflect though.. its defense is a bit, er, lacking to say the least..
Now the big issue is what remaining moves to choose.. Obviously Wish/Protect won't work with this set, maybe Draining Kiss for recovery (+CM boost) and Shadow Ball to hit the steels?