I got live rock and sand from TBS and think brittle and serpent stars are pretty hardy. You get them as part of the second part of your TBS package.
I don't think "true" starfish are ever good for the home aquarium. They slowly die.
Other good fish additions are a yellow watchman goby and pistol shrimp, a lawnmower blenny or an orchid dottyback.
Good luck.
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