nothing that eats algae or pods goes in the refugium.
don't go crazy with a cleanup crew. your tank will do good or it won't, depending on you, and a bunch of tiny inverts aren't going to have a say in it.
nassarius snails eat meaty foods. if you have a problem with uneaten food on the sand bed, they will be fine, but if not, they may have trouble staying fed. they also like to steal food out of the mouths of corals, so if you like feeding your lps, don't get too many of these.
you have the brittles anyway. they should keep the sand bed free of uneaten food.
hermits kill snails, so i would also use these sparingly, or get 20-30 of the little tiny ones. if you are going to do that, why bother getting any at all. when you think about it, the stomach mass of 50 hermit crabs doesn't amount to hardly anything, so i don't see how they can do a good job "cleaning up", and if they did, where is it going? it's getting pooped back out again. they don't "clean" anything.
snails are ok for the tight spots here and there i suppose, but they do a poor job on the glass. you can't rely on snails alone- you will need to scrape the glass, so i ask, why have them?
if you like these creatures and want to have a few here and there just to watch, then by all means, buy some, but please don't go by the "standards" and buy X amount of snails and crabs per gallon.
good water parameters, flow, and a mag float is the best cleanup crew you can have.