THIS. I really want to do this. Someday I'm going to set up a macroalgae tank and stock it with all the tiny things you can't keep in most communities because of their size.
Greenbandeds are fine with others of their kind. My LFS has a 14g reef on their counter that's been up for a few years, and there's two or three greenbanded gobies in there, just fine with each other.
Masked gobies are hover-y fish. When I first saw them, I thought they were baby cardinalfish. Get a group of those, there's no way they can eat anybody else. They act a lot like cardinals, from what I've seen of them.
Perhaps consider a disco fangblenny? Very peaceful fish, and they're goby-like. They hover, too, so one of those could take up a bit of midwater space.
Hector's gobies are lovely fish. You just have to offer nori or something like that now and then, they're almost like blennies in their diet.
Neon gobies and gold neon gobies would be nice. They aren't incredibly long-lived, but you'll have them for a little while.
You need some kind of tiny shrimpgoby and a mini pistol shrimp. This would be a good tank to keep one of those rare-ish shrimpgobies in, given that there's nothing to eat them.