I do not recommend a sand sifter if you are using a functional deep sand bed as part of your filtration. They do eat the beneficial critters in the sand, ruining the efficiency of the sand bed to filter, and can starve to death once they are depleted, at about the 10 month to 1 year mark. Certainly do not keep more than one in this size tank, for best success at keeping it long term. Another drawback is that they tend to stay buried.
Serpent and brittlestars are good choices, except for the green brittle, which can and just might eat small fish. All should be spot fed, and not left to scavenge, regardless of what the LFS says. They should not be put into tanks with large predators, like triggers and puffers...there is a risk that they will be eaten, as these are natural predators.
Chocolate chips, generals, red africans are all tough seastars, but none are reef safe. They should be spot fed with meaty foods.
The reef safe stars including Linckia, Fromia and relatives should not go in a tank younger than 6 months, at least. They can not be spot fed, and must have a large mature tank with a lot of LR, which they feed on. Best success is with a single specimen. Most tend to die at about the 1 year mark of starvation.
I agree that you should wait quite awhile after cycling the tank before adding a seastar of any kind. They are very sensitive to any fluctuations in water parameters, so the tank must be very stable. Specific gravity should be 1.025-1.026, stable pH, etc.
Acclimation is very very important, and should be 4+ hours minimum, for best success. A drip method is easiest. Many stars die within a week of introduction, from acclimation stress. The most delicate are reef safe stars, and the bright red serpent star.