If you want to try shrimp, peppermint shrimp are a great shrimp to start out on because of there cost and overall friendlyness to your other inhabitants.
Just about any small hermit crab will do...a variety of colors is often nice to look at.
astrea, magarita, turbos, and nassarius snails are the snails I have kept. Nassarius are probably my favorite.
Sea star(star fish) are also great at cleaning.
As far as the mixing is concened one could go by inches of living animal flesh per gallon of aquarium water. I prefer viewing it as an art form i.e. "you will know when there are just enough ----".
take it slow add an item every 1-3 weeks. use this time to decide what you really would like your tank to become.
