Just having one in the tank will most likely cause issues for your clowns, I've seen four stripe damsels take nips out of fish 3 times their size, and kill fish the same size as them. My friend gave me the ones she had (3 of them) to be food for the mantis shrimp I had at the time. They had terrorized her tank so badly she didn't mine giving them away to be eaten. However much to our surprise the little ba$$tards terrorized my poor mantis!!!!
I ended up rehoming them.
FYI a 75g is way to small for a regal tang, they get big fairly fast and need a minium 180g.