There aren't a lot to choose from in that size, really. Go to liveaquaria.com and check out the nano section of SW fish. Most of those will even say that 10g is minimum, Unfortunately. The yellow watchman goby doesn't get very big and when paired with a pistol, it doesn't really swim around much. It'll just hang out near the opening of the burrow. The Catalina goby stays quite small, but needs cooler water, like 68-72*F. So if you got one of those, you could do some margarita snails, which are also from cooler waters, but that would be about it cuz most other SW livestock needs warmer water. And coral wont tolerate that temp.
The green citron clown goby is very pretty and colorful. I've seen on Here that some people have toadstool leathers that the clown goby will sit on, and it'll watch everything from there. And then you could do some corals to make the tank more interesting and colorful.