I've got a pixie hawk, and I must say, it has been a trying experience.
He kills ANY fish I have put in. Just continues to terrorize it until it dies. Doesn't matter the size of the fish, he goes after it.
I put in a cleaner shrimp, before I knew about hawkfish. 5 minutes. The amount of time my shrimp lasted.
Any small hermit crab that Hawk can get his mouth on, he eats. He leaves the bigger hermits alone, but he is eyeing them.
Honestly, I think he is a really cool fish. Very personable, knows you. He will swim to whereever you are looking at the tank and just looks at you. My wife likes him, but I am going to trade him and my anemone crab (eats anemones, go figure) in, for some community fish.
Overall, a cool fish, just expect that you can't have certain creatures with them.