From reading, reading, reading and the little personaly experence I have now gathered I'd say you need a good mix to do different jobs.
Snails ceirth/nass are good for sand, astro/turbo for algea(I love my mexican turbos but their large becareful with corals because the can bump stuff off)
Hermits, out of the three kinds I have: drawf blue legged, scarlet and tri-coloured legged I find the tri-colour to be the most peaceful. None have killed anything but the scarletts and blues do switch up shells a lot and I'll seen then kind of fight but nothing serious.
I've herad sally light foots and emeral(sp?) crabs are good but no experience. I have two pepperment shrimp but5 I haven't seen them yet gggggrrrrrr I want a blood fire shrimp as they look so cool and a cleaner shrimp because they seem to be the most popular for a CUC.