I'd stick with nano fish, as the small footprint is not big enough for your average tetra/rasbora/barb. To be completely honest I wouldn't do any sort of active schooler in that sort of footprint, but you could mayyybe do chili rasboras, CPDs or ember tetras, but they'd be much better off in a longer tank like a standard ten gal
(same goes for the gourami). Actually, yeah I wouldn't even do any of those little guys in a cube. Your tank sounds like a great size for a betta, dwarf puffer, or a shrimp tank though.