I had actually purchased a small sand-sifting star for my reefbowl but decided against it because if the animal were to ever die for any reason, it would wreck my small tank quickly as its unskimmed.
Also, if it burrows under my rockwork it may dislodge it or cause a shift--this animal it seems would be quite hardy and fine for a ten gallon but not for a one gallon. The bioload was not my concern, there is enough live rock and circulation in my pico to quickly nitrify the wastes from a small echinoderm--I just never wanted to have to tear my rockwork apart to dig it out in the future. This is also why I never bought small brittle stars either.
Small crabs stir the upper layers of sand, but there are claims the decimate small fauna such as serupulid or sabellid worms.
I have a small boxer crab who does not seem to do this, and he readily picks through dinoflagellate layers on the bed to keep them from building up. For your size system, Id vote for a small sand sifting star (gen/sp?)
HTH
Brandon