I'd be careful with some crabs, esp arrow. I've got one and I'm not too sure about him. The others, emerald, decorator and annome don't bother anyone at all. But that arrow.
I'd go with less agressive shrimp, so no cbs. Maybe pepermint? I can't see a snail or urchin bothering a seahorse either. I'd with the urchins that can suck to the glass so you can feed them easier.
I've heard of some stars eatting small or slow or sleeping fish. Keep the star well fed.
Edit, caught the emerald carrying the clam today. Not sure if it did any harm, or what it was trying to do.