Yes, you will need a crew. Scarlet hermit crabs are good and some assorted snails. Not sure how many though. I forget how many I bought when I set up my tank, I think I bought like 30 scarlets and about 60 snails (Narcisus? spelling they're real small and bury themselves in the sand bed). You could probably cut those in half with a 55 gallon to start (12 hermits and 25 snails) and add more if you need too. I've added more to replace losses.
To get a better idea, look at some of the reef packages on this site. They break it down for you with suggestions.
Emerald crabs are nice but I would get just one. I have three in my 90.