Hmm, you ask an interesting question. Ideally you are going to want to stick with smaller sized fish. You could probably do some sort of pygmy angel like a coral beauty or flame angel. Although it is not a guarantee they will be coral safe. The other part of the equation is the damsels you have in there now. They usually get temperamental and territorial. Another option might be to get a small school of green chromis. while it is true a lot of people have them they do look cool I think. And it is fun watching them all swim around together. When they hit the light just right they almost turn a blue color.