Snails- Nassarius for stirring the sand they are scavengers that burrow in the sand. Astraea eat algae and hand on glass and rock. Mexican turbo snails are very large and kick ace at eating algae on rock, glass, and sand. Cerith snails are small and burrow in the sand and eat algae on sand, glass, and rock. You can get away with a lot of snails but I'd start at 40-50.
Starfish- need a lot of sand and a larger tank. Your 75 would be great for a few sand sifting starfish but you might not be able to add them until your tank matures.
Crabs- more people are starting to get away from hermits as they terrorize crabs b/c they want their shell for their new home. If you get some make sure you have a lot of extra shells for them to play in. Blue legs are notorious for killing snails. Scarlet hermits are the most docile of the snails but are more lethargic than the others. I have a mix of scarlets, red legs, and blue legs. My red legs and blue legs do the most work. You can get away with 40 snails I would think.
That's the limit of my knowledge. You can look into seahares and urchins also but there are even more options out there.
