I was given a 20 gallon saltwater aquarium recently. The are 2 Common Clown Fish, 2 Blue Damsels, 1 Neon Dottyback and 1 Ocellated Dragonet. The all are healthy and appear to "get along" fine.
I wouldnt add any more....but what matters really is water quality. How well the tank and filter are handling the bioload.
Have you been testing regularly? how are your phosphates and nitrates? If they are staying stable with normal water changes, your good. (least with these fish none are open water need lots of swimming room)
Those damsels don't get too bad as far as aggression. In fact, all the fish, except the dragonet, are all the 'same speed' as far as aggression goes. I don't think it will be a problem, though they probably won't allow any new fish.