It's... kind of weird. We used to have a policy that only the spriters themselves would be able to do shinies, as they kind of wanted to, but it's been so long most of the spriters don't really frequent the forums much anymore, so yes, anyone can make shinies.
As for which ones are allowed, I'd say all of them. We'll just choose the ones we like best at the end (a good idea has been to put a red nose on Caribrisk). We're not implementing them for the demo, so there's no hurry.