Hermits are great if they are small. If they have any size to them they will kill snails like its a sport!
Cerith snails, Astrea snails, Nerite snails, Trochus snails, Nassarius snails, red legged hermits, blue legged hermits, cleaner shrimp of some kind(peppermint, skunk cleaner, fire), brittle stars, sand sifting stars, cucumbers(not recommended, can foul a tank up if they die), emerald crabs, sally lightfoot crabs, urchins, Fighting Conch snails.
All of the above are reef safe and contribute to cleaning the tank in some way. The crabs can get big and aggressive but when they are smaller should not be a problem.
In a 55g it will be hard but if you plan on upgrading to something bigger(120 or bigger) in the next year or less you can start off with some small tangs(2" or less in length). They graze for algae and will also help keep the tank clean. I have a Yellow Tang, Regal Tang and Powder Blue Tang in my 55g and they are just fine but I will need to upgrade to a bigger tank in the next 6 to 8 months because these fish can get aggressive and they grow fast! If you aren't planning an upgrade then you can sell them back to your
LFS when they get too big and get credit for different fish.
What fish are you planning on keeping?