Clown question

[Gilbert Lowe]

I had two ocellaris clowns in my 12 gallon for 9 months. I recently had to give one two my LFS because the other was attacking it and it jumped out of my tank a few times. Now I have the one fish. Will I be able to add a clown to the tank again or will this clown just beat on the next one I put in. Is there a period of time I should wait before adding a fish? If I can would I be able to add on Onyx or true perc? Do I need to stick with the Ocellaris?

Thanks