Spotted rasboras - rasbora maculata. A small school of these would work beautifuly. Alternatively, you could try neon tetras.
Pygmy cories (c. pygmaeus but there are other small species as well - c. hastatus for example). Again, a small group would make a lovely bottom-dwelling addition.
If you'd be interested in livebearers, either a trio of mosquito fish (heterandria formosa - NOT gambusia affinis) or a trio of endler's are fish to consider.
The myer's hatchetfish (carnegiella myersi) also grows to only about 1" and would make an excellent, peaceful, schooling top-dweller if you could find a few.
Obviously there are more gourami species that stay small but I'd guess you don't want to put any more in there and risk over-crowding.
All the above are compatible with each other and with your fish. Just make sure to keep them in adequate numbers/ratios and to not over-stock. Most would also be compatible with the clown killi and all would be compatible with the glowlight tetras which were both mentioned earlier.