RE: tang compatibility
I would not put regular damsels in the tank. My tank was set up with damsels and then I had a two month process of getting them out because I wanted to add cooler fish that they would just pester (even fish bigger than them).
The idea of putting some dither fish in has merit. I would go with a small school of chromis (which are in the damsel family, but much more docile) or maybe a small school of pajama cardinals. BTW, what are the other fish in your system.
144 G Half Circle Mixed Reef, 240G FOWLR