removing the damsels would help.... tangs aren't a problem in your 55g as long as they are ones that are small & stay small & also do not have a ton of energy.
Naso, Vlmingi, Sohal & a few others get big with the later 2 would outgrow the tank in months so they are a no
Powder Blue, powder brown, achilles & a few others have way to much energy for a 55g & wouldn't have enough room to stay busy
Blue Hep tangs can grow fast but do not swim a lot so they would be fine in there. Same with a yellow tang, not active swimmer..... they just have a big attitude
the hep/hippo tangs are also ich magnets, not nearly as bad as a poweder blue but a lot more sensitive then the yellow