Just weighing in here. People have mentioned linckia (beautiful but delicate), brittle/serpent (really freaking cool and easy to feed), chocolate chip (predators but I know little about them), sand sifters (unfortunate habit of starving and hiding under the sand to die), but no one has mentioned fromia stars!
These guys are gorgeous and not as delicate as the linckia. Not as hardy as brittles but still good. I had a marbled fromia for almost a year before it stupidly crawled into a bubble coral and was stung to death.
I can't recommend sand sifters at all, and linckia are not really the best "first sea star" to buy. The chocolate chip and similar are good for FO
, but not reef. Serpent and brittles are great and I love them. Except for the green brittle, I'd say odds of attacking anything mobile are low.
But please dont discount the fromia. They really are beautiful and if you have enough rock, a 50 *should* be big enough to accommodate one.