You could try bartletts or squampinnis (lyretails). Personally I have had no sucess with dispars - couldnt get them to eat.
Best bet in a 55 is to just add one, however, if you wish to have a group I would add 4-5 females and let one change.
I have a group of 9 squampinnis (1 male) in my 180 gallon reef and they are doing really well - have been stable as a group for over a year now.
Simon