Nassarius snails or a olive snail would be fine to clean the sand for a closed system of your size. Have you had a diatom outbreak yet? Also I would stay away from Damsels. They are real aggressive for a tank that size. If it were mine I would get a pair of clowns (of course), a six line wrasse or a royal gramma, and a small goby like a yellow watchman) Seems like you got a decent clean up crew. Maybe a very small serpent starfish. They are real cool. ;)