I'm not sure about red serpent stars, but I have 2 huge red brittle stars in my tank with all my fish and haven't had a problem. My brittle stars have bodies that are about 1" in diameter and legs that are each about 5" long. So they're about 11-12" across. I have a YWG, 2 firefish, 2 clowns, a coral beauty, and a midas blenny. I also have 2 cleaner shrimp, a pistol shrimp, and lots of crabs and snails and I've never seen either of the big stars mess with any of the other inhabitants. I do have 3 serpent stars but theyre much smaller and not the red variety. They're model citizens as well.
If you would like to get brittle or serpent stars that are smaller in size so that you don't have to worry about the potential for them eating fish, KP Aquatics sells "nano" sized brittles and serpents. The best part is they ship just fine with priority shipping so its only $15 to have them shipped. All 5 of my stars are from that website and I know a lot of people who have had great success with stars from them. Their website is:
http://www.sealifeinc.net/