1) Your sand sifter star stands a high chance of dying of starvation in 9-12 months, and is counter productive to cleaning the sand. It does not clean the sand. It stirrs it while consuming the critters you want to cultivate that clean the sand. When they are gone, the star slowly starves, unless if takes to spot feeding (not many do). I would return it or not buy it.
2) The chocolate chip star is not reef safe and is a threat to snails, clams, corals and other slow moving critters. Easy to spot feed, and relatively hardy nonetheless.
3) Serpentstars should not be left merely to scavenge and should be spot fed. They will do some "cleaning" but not a lot of "waste" (excess food but not poo so much). The green is a known predator, and others may (or may not - mostly not) display this behavior. They are nice interesting additions to a tank, but, IMO, should probably be added because you want one, not because you want it to do a job.
I would stick in that case with sand sifting snails such as ceriths, nassarius and conch.