I just got a flame angel about a month ago and dealt with the same concerns....
Experienced flame angel owners will say that "reef-safe" flames are hit-and-miss depending on their specific personality and appetite. The one that I got seemed really friendly at the LFS and exhibited an interesting personality, so I decided to take the gamble.
The first two days, my flame angel was busy warding off my kole tang's territorial attacks. Since they were about the same size, it was difficult for the kole tang to share the tank with the new flame angel. After two days of battling for water rights, they finally threw in the towel together and became friendly tank-mates. I was really afraid right from the start that the flame would start pecking at my soft corals (I have xenia, zoo polyps, a torch, and some star polyps).
Sooner or later, I noticed the flame angel always "hung out" near the rock where the xenia was planted. It soon started to "peck" at the tips of my xenia, making them shrivel into their base. My solution was to have two small feedings a day rather than one feeding per day. So far, it's been working, even though the flame still tends to peck at the xenia when it's hungry. It's basically the signal for me to feed.... =)
I don't mind it as much because flame angels are one of the most brightly colored and beautiful fish IMO. My flame really doesn't bother any of my other corals... my further research on the internet resulted in finding that flames WILL favor xenias and similar corals over others. As I'm typing this reply, my flame is starting to PECK, so I guess it's time to FEED!
Hope that helps!