a couple of thoughts for you.....not sure what "crabs" you are thinking of, but if they are hermits then they will not play nice with your snails. Pick one or the other IMO.
Emerald crabs are hit and miss for valonia so if you are getting them for that just know going in they may or may not work.
for sand sifting you have several choices; gobies work great, but they will dump sand on anything below them....cucumbers have been great for me and I highly recommend them to keep the sand clean...sand sifting starfish can work out, but it is hit and miss.
glass cleaners - I am assuming you are thinking for diatoms on the glass, in which case astrea snails work well, but their life span is quesionable.
As far as what else and how much.....how much depends on your tank size, your algae level, what algaes, what fish, etc. I like my hermits, but would not mix them with snails without expecting losses in the snails. Some argue that if you provide empty shells they will leave the snails alone, but my experience says they like a tasty escargo every once in a while.