Get the generic cleaning crew (hermit crabs, more snails, shrimp, maybe a brittle star or urchin), as far as more exotic inverts go, wait a while before you add any. 2mo's is still a very young tank. Your water parmaters maybe fine now but they may not have been when you added the damsels, that why those brave pioneers where recomended as starter fish, they deal with the harsh conditions in a new tank better then others. Even then that is sometimes not enough. Not to say new tank conditions is defintly responsible for your fishies demise as there are to many variables not accounted for. Bottom line is that your tank looks good now, and if you have a yellow tang healthy (even if he is scared), it should be safe for other comparable fish. Just stay away from the delicates for the time being (butterfly's, Angel's, other tangs...) and you shoudl be alright. And as the preivous poster said, no more then 1-2 a month.
