Thank you all for the feedback!
I didn't even think of Fittonia. I have a fairly large one as a houseplant, I guess I forgot they come in small too.
I am aware of the size constraints but I think I can make it work. All of the fish on my "maybe" list are under two inches and are happy in pairs or small groups.
Do you think lava rock would be too abrasive for burrowing animals? What about flourite?