May I mix and match armour?

I’m sure the answer is no. But the Q is.

If I have the celestial helmet on with dragon body armour. Do I get both armour bonuses? And if not which one counts?

Common sense tells me body.

Cosmetically you are free to mix and match armor pieces. This doesn’t affect your bonuses. Your bonuses are entirely affected by the armor you equip. Think of it like “transmog” or “glamour” systems in other RPGs; you get the stats of one set of armor and the appearance of another.