Most hitchhiker crabs are bad...they will either eat corals or fish. What I'm picturing in my head of your description of the divider won't work. I'd pull them all out now, and get a positive ID. Nothing with claws is allowed in my tanks (hermits, crabs, lobsters), with the exception of Peppermint and Cleaner shrimp. Shrimp are really good for eating uneaten food, and won't cause trouble (other than maybe the Coral Banded shrimp).
Personally, I run RowaPhos (in a reactor now that I have one) as soon as the cycle is over.
You can clean your glass of the sponges. I keep my glass all nice n clean.
Don't worry about them being on the rocks or anything, they will die down once your tank establishes itself better.