I usually don't post on here but i thought i would throw in my two cents because I set up a tank very similar to yours 3 months ago. I think the powerhead is something you definately do need. I have an AC110 fuge and a maxijet 600 in my ten and i am considering adding another small powerhead to get a little more movement. I have a pair of clowns and some easier corals, softies, zoas, hammer and a small acan frag all of which seem to be doing fine.
You also might want to reconsider the purple firefish, from what i have read they are pretty peaceful and in a tank this small the clowns are going to make the entire tank their territory so they might beat it up. If you want some more life get a cleaner shrimp, they can be really entertaining.