Are your percs still juveniles? You may not see any problems until they mature. Especially when it comes time for them to lay eggs. They might pair up and start fighting for territory, or theres a chance they might develop a pecking order in that case the weakest clown will be bullied. Theres a good chance you will wind up with another dead clown.
Personally I always suggest just one pair of clowns per tank. I think they're happier to have their own territory.
Your going to get mixed opinions about the yellow tang. Some people say you shouldnt have one in smaller tanks. Some people say its ok as long as you get a small tang, and either upgrade your tank size, or give the fish away when it becomes too large for the tank. You have to do what works for you, and keeps your tank healthy.
Good luck I hope everything works out!!!!