I have 40+ hermits in my tank and haven't had an issue as of yet.. They clean the algae very well.. I know that at Etropicals if you order 60.00+ of their tank cleaners its free shipping.. Awesome deal.. I like the Blue Legged hermits, Red tipped, and the Zebra hermits..
As for whelks I tried to even search using Copernic Pro and wasnt able to find much.. You might want to check Clamsdirect.. Whelks are clam predators mainly and you might be able to find some there.. There is a link in my sig to the clamsdirect website..