I'd second the firefish for something that darts in and out of rock work. There are 3 kinds to choose from. Just be aware that they like to jump, a lot.
A fish that swims around a lot that might work are the Meiacanthus blenny's. I have the Smith's Blenny and it is always out. Another option is a Percula, Occelaris or Skunk clownfish. They all stay relatively small and if it is a single one, it shouldn't get aggressive. If you get 2 and they start to breed, they could get aggressive during that period. Most other types of clowns get too big and aggressive. Clowns are always out and if you get xenia or fuzzy mushrooms, they might host that. Then again, they might host your powerhead as well like a pair of mine did.
