Brittle or serpent stars will be good. I have a brittle that I have had for two and a half years. Brittles and serpents are very close in their nature and care. They are typically scavengers and will eat stray food or dead or dying fish/inverts. Some claim that they have had them eat healthy fish. Particularily with the green brittles. I don't know, I never seen it.
If you spot feed them they should get plenty of food and not need to resort to eating live fish. If you starve them, either by not feeding, or if you tank does not have enough stray food, they could plausably resort to finding a live fish.
But, do not feed them so much that they will not scavange!! Part of their charm is the fact that if a fish does die and fall behing the liverock where it can not be reached, the star will go and clean it up for you, before the it spoils the water.
It is said that Linka Multiflora will survive and sometimes thrive in tanks as small as 29 gallons, but they still need ample liverock to feed off of. I would think 20 lbs may be too little. I would try for nothing less than 50 lbs to be safe.
All other linkias need lots of liverock and well established systems to have enough to feed of off.
Sand stars are good too, but if you have a deep sand bed, as opposed to crushed coral, the sand star will destroy and eat all of the critters that keep you sand bed functioning. So not good for dsb's but good for crushed coral. Also, sand stars need a lot of critters to eat so make sure you have plenty of critters in your crushed coral to keep them fed. Some have had luck spot feeding them. I did not.
Also, star fish typically like salinity around 1.025 - 1.026, and need to be acclimated very carefully. A drip method over many hours (at least 4 but more is better) is recommended.
Do some searches as stars have been a hot topic. Also, Ophiura is considered the resident starfish expert as she studies them professionally.