I second the "brittle stars" eat fish concern. Think of brittles as the green prickly ones or other ones with spines all over them. "serpant stars" would be the smooth ones. I usually see green brittle stars, (black) brittle stars, or black and tan banded serpant, tan serpant, orange serpant as trade names in the shops.
If I was you I would go out of my way to find an orange serpant star. They look really cool and I would assume that they behave the same as other "serpant stars". Someone, I think it was Leo, said they ship poorly so use caution. Otherwise the more common serpant stars are drab looking but great reef inhabitants.
Any of the above (including the brittles) are really interesting to keep, they will smell food and come running (as a starfish runs) in a matter of seconds. When I had green brittles they appeared to mate a couple of times. Nothing came of it but they were clearly, um, hugging pretty seriously.