How big is the tank?
Clown tangs will dominate any other tangs. They won't "get on", the regal tang will know it's place as soon as it sees the clown tang.
Clown tangs need big long tanks and incredibly high water flow though, so make sure you have this or it'll experience muscular wastage and ultimately die of lack of exercise. They're fish that live right up at the top of the reef in the shallows, they're built to fight massive currents to avoid being smashed into corals, and without that flow, they waste away.