What kind of filefish? These are considered cousins to the trigger, they can have a mild temper unless provoked, when provoked they can turn on a fish.
My tasseled is boss of my Pinktail, my Pinktail will back right down when the file wants to challenge it. If you look at the Tasseled than you can not have any nippy fish in with it. If you look at the smaller filefish which would be best in your 55g, they can be kept with most smaller fish.
I have kept filefish with chromis, clowns and so on yet it never harassed another fish unless challenged..... Hope this helps
Kaye