Perhaps it was a bit of mislabeling, and definition of brackish.
Rift lake (Lakes Malawi and Tanganyika) african chiclids do like some salt in their water, I'm guessing you used 1 tablespoon per gallon. I think they sell special salts for this, but I don't know how important the specific salt is. From your description, clownfish, you DO have Rift Lake African Chiclids, no idea on species, there are a lot.
Other "Brackish" fish come from areas where river or bay meets the ocean, so the water is more salty than pure fresh, but not full salt like the ocean. Those areas are where you would find the species listed above by Terry.
Back to your original question, sorry, really not sure what type of cleanup crew would be good in that tank. African chiclids will eventually destroy any crabs you put in there, I do know that.