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Get rid of the damsel which will likely harrass any newcomer since it's HIS home now already. Look at clowns, blennies, and gobies. What sized tank byt the way. That'll help with recommndations on what next.
Yep. Oh yeah I almost forgot.... "duh" - he did say 90g.
Still, I agree with David. That damsel can cause more problems than he's worth. Newcomers, regardless of size may get harrassed to the point of sickness, infect others and everybody can die. Worst case. If he does harrass others, you'd have to remove about 80 to 100% of the rock to catch him. tearing up the tank can cause a whole new set of problems.
I agree, get rid of the damsel, or get a QT going put him in there for a while. Get the new fish into your DT and then add him after so he is the "new" guy. But this is a lot of work for a damsel. Sell him and use the money to buy some clowns!