Im a fan of brittle stars and tons of bristle worms. They clean up alot of gunk. I dont buy hermits anymore they tend to be jerks and murder snails so they can play dress up with different shells. I like trochus snails they tend to have an easier time flipping themselves over if they fall off the glass. Turbo snails are nice but they need attention cause if they fall over they die and their rather large size can upset the water if you let them decay. I like peppermint shrimp and scarlet skunk shrimp. Id stay away from coral banded shrimp they are notorious for being aggressive. Id stay away from sand sifting starfish until your tank is old so the star wont starve. The nessarius snails sift threw the sand but they also eat meat and will come up and steal pieces of food you feed your shrimp or brittle stars. (Atleast thats what mine do)
I would stock lightly in a clean up crew until you know that they need help because if you over do it then you are just slowly starving them to death.