Damsels are rather problematic in small spaces, although primarily with things that are related or similarly shaped - but they will still go after just about anything if it comes too close. If the damsels are claiming the whole space as "theirs" right now, they might not leave a new fish alone. It's also common in small tanks like this one for groups of damsels to harass smaller individuals to death over time even if they get along reasonably well when young.
Aside from species, you've got a lot of fish per volume of water for a marine tank right now. Unless the tank is sumped, you may run into overstocking issues as far as nutrient export by adding anything else. Even if not an immediate problem, it can become a problem down the line if the fish are not full-sized right now.
Really the only option if this has started is to thin out the damsel population. Also, if you take the injured one out, it's entirely possible another one will show up ragged right away since the aggressors will keep looking for things to go after. Injured individuals will usually keep getting injured by their attackers in little bits here and there until they die, and then the attention turns to the next weakest fish. Some damsels can be quite resilient and survive months with constant harassment, but it's just survival under stress - they won't thrive.
One of mine is now missing most of his hind fin! This of course worries me but I don't want to get ride of them. Isn't there anything i can do?