I don't recommend any tangs for anything under 75g and even smaller spp. require enough space to comfortably move in and around decor at max size without having to swim at awkward angles or run into objects (which usually is the case). Tangs are territorial and it matters not how many are in a system, they are going to bicker; I've seen single tang tanks chase after wrasses, clownfishes, triggers, etc. Someone mentioned ich, of which it won't get ich if you QT properly. Most of the time cramped quarters, due to territorial encroachment and inability to maneuver at max size, brings upon infections from either bites or wounds. Green damsel? You mean Green chromis, Chromis viridis? They would be fine if you can find healthy stock. If you Search this site I've mentioned a few times of damsels that are fairly peaceful, ones I wouldn't think twice about having in my own system 
