1. Just caught Zygarde 31/29/30/x/31/31, but unfortunately its nature isnt synchronized, which is Careful + SpD -SpA.
Should I run DD or Coil on this one?
2.Does Refrigerate boost the move's power like Pixilate?
I don't have an answer for number 2, I'm afraid, try testing it by using it in game and checking against some calcs, is the best I can suggest, if it's not been confirmed anywhere.
However,
Number 1: It really depends on what sort of set you want to run; if you want some all out bulk, and EV accordingly, then Coil will work well; he has access to Extremespeed and Dragon Tail, and a few other moves that fit well on a bulky set. However, just be careful in that you basically need to be able to force a switch to set up Coil and sweep with it, for lack of reliable recovery; if you can get a DD off, then you'll outspeed (and therefore threaten) a whole lot more, which can make a bulky EV spread and nature all the more threatening too. Don't despair at his nature; a specially-defensive EV spread on Zygarde might be viable, considering he's already got reasonable HP and great defence.
EDIT: I'm going to add a question of my own - is there a place for early-look 6th-gen teams to be posted up? I've not found it yet, and I've not seen more than one or two here, but I'm keen to see what people are using and to post mine up too.