A diamond goby is probably your best bet. They do a wonderful job at cleaning the bottom of the tank.
Sand sifting stars, cucumbers, and mexican turbo snails are all poor ideas. Both the sand sifting star and sand sifting cucumer need very large sand beds. Sand sifting stars usually starve if they are not in a large tank with a very deep sand bed. I would recommend putting them in at least a 125 gallon tank with a 6" sand bed. Sand sifting cucumbers need to be in a very peaceful tank, because if they feel threatened, they can release thier toxic insides, killing the fish in your tank. Mexican turbo snails are not a great idea because they are mostly rock cleaners.
If you are looking for some type of snail, nassaurius and cerith snails are your absolute best bet. They are strictly sand cleaners.