I think Turbo snails may be your best bet for the diatom algae. If you ask me, thats all they are good for.
And I would guess some astrea snails also.
Sleeper gobies are many times a hard fish to feed. Try find one thats eating prepared foods. Mine use to eat anything I put in, esp. frpzen mysis and so on. One good trick is to use some Marine "A" pellets and place them around the bottom. The gobies will eat them there more readily.