I think that your 55 gallon will be plenty enough water volume to keep such a fish. For example, I owned a clownfish and I put him in my 6 gallon nano cube, despite what everyone told me. I stuck to my weekly water changes and guess what: no problems at all. In this hobby, they only recommend you to keep some fish in larger communities because it will make maintenance easier on you. If you are devoted to the hobby and have a well cycled tank, I say go for it and enjoy a colorful fish. If it bothers you to keep up with your tank, then you may want to go with something easier, but the flame angel should not give you even the slightest trouble.