It depends on a lot of things. How often are you willing to do water changes? do you have any nitrate sensitive inverts? What are your oxygen levels?It sounds like you have about as many as your tank can handle right now, but I understand the desire to pack our tanks with fish. There are just so many interesting creatures out there!
If you add something new, keep it small and hardy-a lemon goby which perches on the rock would be nice-and add a different color.
What about a brittle star, some shrimp, snails etc. which are extremely interesting and all serve a very useful purpose?
Check out the fish and inverts on this web site. There is a great selection.