You can have several species of brittle/serpenstars in that size tank easily, but avoid the green.
I have 10 in a 45 g. In the wild, you will commonly find several under a single piece of rock.
Your sand sifter will likely starve in 12 to 18 months. You will probably think something is eating it. The arms will start to disintegrate. You can try spot feeding, but most will not take to it. Try anything meaty.
Carnivorous seastars can be kept several to a tank, but they will eat snails, corals, hermits, etc and possibly even each other though keep them well fed.
Reef safe seastars can definitely be too much for a single tank. IMO, even a single one would be marginal in most 55g tanks. More would likely eat each other into starvation.