Zygarde-C an alternate form, so it most likely will revert back to its original form upon mega evolution. Whether or not Zygarde-C will be able to hold a megastone at some point is something the council for Mix & Mega have to discuss. It'd be pretty cool if it were allowed, but I'm not sure if it'd be broken or not.Would this also theoretically be the case for Zygarde-C, i.e. would a Zygarde-C with a mega stone (if that were allowed) revert back to regular Zygarde before adding the mega stone stat changes? If that is the case, would it be an option to allow Zygarde-C to use mega stones, since in some sense it is just a regular Zygarde with a mega stone?