They should be alright as long as they have plenty of space to move about and plenty of holes to live in, and of course a tight fitting lid to keep them in the tank.
I would also recommend asking questions about this sort of thing before you put something in the tank
The snowflake is eventually going to be a problem. They eventually get big enough to eat the flame angel. But are also going to potentially knock around corals and rocks once they get to adult size.
They should be fine, what size tank, they only get about 2' and will not even look at that flame. The only time I've ever seen a snowflake cause problems was when a pretty large one right from the ocean was added in a tank with inverts and very small fish. That one looks pretty small, by the end of the week, the two of them will likely be cuddled together.