In terms of looking for a CUC and size, check out reefcleaners.com. A great site. I've ordered a couple sets off of him, always quite numerous. The thing about CUC's that I've learned is that every 6 months to a year they need recharged. This can come from hermits picking off snails or just natural death as breeding doesn't always occur in our tanks, even though in setups like mine they are always covered in snail eggs.
But in my 55, I try to have about 60ish snails in there at one time and 20 hermits. I have a serpent star, bristle star, and a fire shrimp as well, but they aren't something needed just optional.