Ordinarily I would say fine. But that's a small tank, it might be better suited to the hawk than to the flame. At any rate, I had a five year old well fed flame angel in a bigger system. Napoleon ruled that tank, but I figured a flame hawk shouldn't bother him but it did. He made it his mission in life to restrict the hawks movements to one very small corner of the tank with a coral colony. He did this for a year and I had to rehome the hawk.
While I believe this behaviour was atypical, I think there are a few things that contributed to it; I don't think the similar colour had much to do with it. Firstly Napoleon's age and status in the tank. Their behaviour changes a lot with age, so your best chance is with a younger fish especially when introduced to an existing fish's domain. The other thing was I wondered whether running the temperature high (27C) contributed to the aggression. Not sure, but I've since noticed aggression can be curbed in part by running a lower temperature. Finally, I think if he had to interact with another angel and the corresponding mental stimulation he might have paid less attention to the poor hawk.
Just some food for thought,
:wave: