I would not keep any crabs with them, I also would not keep peppermint shrimp with them. Zoanthids and ricordia, as well as other corals, can introduce dangerous things to the tank, but provided you are careful, I think you could try the zoanthids... But, if you have ever treated the tank with fenbendizole, you cannot keep corals, and if you ever do need to treat with fenbendizole in order to eradicate hydroids, the corals will need to be removed first and won't be able to go back into the tank.
I would look into macro algaes, those tiny red shrimp (volcano shrimp?), that are less than an inch full grown, nerite snails and nassarius snails. And, definately keep an eye out for hydroids. They can kill your dwarves and can come in on anything live that you add to the tank.