I thought it was bad to mix any kind of angelfish regardless of tank size. The emperor angel will get kinda big and probably aggressive toward the flame angel. I say pick one or the other. I really love flame angels personally!
Most of the people I know are running 120's-150's for there fuge's for 300-500 gallon tanks now those are very big tanks, now back to the question, if you are going to mix angels your best bet would be with a dwarf angel and a large angel I dont think I would try it but it could work with plenty of hiding places and I wouldnt trry this in anything shorter than a 6 foot tank.