No issues. If you want them to become a mated pair, best to add them both tiny or add one that is noticeably larger than the other to ensure only one female.
Add away! Do you plan on trying to breed them? I have a pair of Clarkii that spawn all the time but since they’re in a tank with so many others, I’m sure the fry get eaten. I’ve never seen any little clowns!
You can change up Ocellaris and Percula varieties in just about any configuration. As long as it's no more than a pair and they are added when they are small or one is larger than the other it should work.