IME chromis fish do fine in groups of 6 or more. In a 55 that would be fine, but your losing stocking of other fish. Chromis form a pecking order, and the more that order is spread out the less likely one fish dieing, and another becoming the bullied, and so on.
Firefish, I've never had luck with multiple. They require a good sized tank and even then they'll be at opposite sides.
Id avoid the eel, thats just me however.
Most butterflies, even though some sites say 50 gallons, should be kept in large well cycle tanks. due to there activeness, much like that of a tang, the longer the tank the better. 55 would be better then anything less, but id still advise a hold on the butterfly.
As big gourami said, if you intend and will upgrade in a few years, a lot of this could be tried, and if successful, then start focusing your efforts to that larger tank.