for hermit crabs i prefer the dwarf hermits and I try to mix it up as far as color for diversity, you can do 1 per gallon of water (other opinions may vary) as for snails I tend to stick to about 1 snail per 3-4 gallons of water, I like cerith snails and margarita snails and only a few super tonan nassarius snails to clean my sand bed
Again opinions do vary on this subject