I agree that your bioload is pretty light right now...especially with types of fish you currently have...not are very messy and do not contribute greatly to bioload. You can stack the deck more in your favor by feeding quality foods in lower quanities.
As for fish...you have several options. I like the royal gramma, sixline wrasse, shrimp goby, or a small school of reef chromis. Most tangs may be out with the exception of maybe a yellow-eyed kole. I would not attempt any of the dwarf angles in your reef set up.
Some people are like slinkies...they serve no real purpose yet can still bring a smile to your face when you push them down the stairs!:p
Have a great day! Brian