I think finley meant zoas. If you have a zoa frag that doesn't seem all that interested in closing up when a bit of food falls on it, then you can try it. It is going to be easier in the long run for you if your zoa is fraged onto something like a ceramic plug that isn't going to harbor any *nasties* (in the dwarf world, nasties are hydroids, which normally don't upset reefers, but they flourish in systems that are constantly fed live brine shrimp -- and dwarf seahorses are -- and the hydroids' sting can be very dangerous to a dwarf.)
As for the starfish, the tiny micro brittles, the white/cream/grey colored ones that are often found in the sand of established reef tanks and have bodies that are no larger than approx. 1 cm, those would be okay with your dwarves and would help to keep your sandbed clean. I have never witnessed those guys use their legs (no matter how long they grow) for anything other than locomotion. I don't think they could threaten even a baby dwarf.
As for the mini brittles, the ones that like to wave their striped arms out of live rock, well, live rock often = hydroids so you probably won't have any of that, and I suppose it is possible that a mini brittle might catch and consume a baby dwarf.
There are tiny snails that you can use as algae eaters, called collonistas, that are often found in reef tanks as hitchhikers. They are almost pretty and in proportion with the dwarf seahorses.
It is likely that neither the micro brittles nor the collonistas can surrive panacur treatments (a pet/large animal dewormer that is sometimes used in dwarf tanks to kill hydroids.)