The only ones I would suggest then is the banded serpent star or
a black brittle star. Being such a young and smaller tank, it will not really support more than one at this point.
I would also not recommend the sand sifter. They are voracious carnivours and will quickly deplete the fauna in the sand bed as well as possibley prey on other stars.
Just remember to acclimate slowly. Drip acclimation
is the best suggestion.