Yup, the only one's that are really beneficial in a sense are the brittlestars (same as serpent stars). These will scavenge, but should, IMO, be spot fed as well. However, the green brittle is a known predator as well. The knobby stars, including the red African, general and chocolate chip will also eat large dead things, like fish, in addition to a wide range of other living things.
The purple Linkcia is not, it is (in most instances) Tamaria stria and reports have come in about it eating a variety of soft corals including mushrooms. This is a danger. We know very little about the diets of many of these sea stars. All Linckia and Fromia and relatives are very delicate, requiring many hours of acclimation. It is best to keep them one to a tank, and the best success is in tanks of 100+ g with at least that much LR, and no other similar stars.
BTW, I have never seen a 'general' star that red. Strange. Where did you get it from?