Hermit crabs grow at an alarming rate if fed well. they are on the constant look out for a better piece of real estate and if your snails or even other hermit crabs shells fit the bill then a large hermit will just take it regardless weither there is someone home or not. Its a good idea to have some spare empty shells to suit hermit habitation just stearlise them and pop them in your tank to deter hermit snail predation.
For CUC in my tank I have 3 Turbo Snails, 8 Star Astrea snails, 4 Cone Astrea snails, 10 Jumbo Cerith snails, 4 Nassarius snails, 8 Blue Leg Hermits, 4 Red Leg Hermits and 2 Cleaner Shrimp.
Hope this was of some help to you.
Regards
BigC