i would get a pair or a trio of sand sifting stars and some nassarius snails...they looks small but they really pack a punch imo. they are always roaming around digging in and out of the substrate
in a 55, you are really going to need more cleanup crew than you have. i would look at at least 15-20 small hermits (blue legs, or the little red legged ones, or scarlets) and the same number of snails