There are a lot of nano gobies out there and barnacle blennies that stay very small and will have a very minimal impact on your bioload. Outside of those I think you are making the right decision to keep it fishless.
I think a Grouper would be perfect for that tank!
All kidding aside, that's a cool little build you have going there. I love seeing things like this that people build for themselves.
aj1080 gave me a nice cheato and a rock cover in mushroom , he's a cool dude . i was thnking a neon gobie or a grouper jk but i think i like invert tank better . i love building stuff for my self i appreciation for it