i'd be interested in seeing how well the duckweed does as a filter, I know i neglected a tank with H. Polysperma in it and it wiped everything else out and made a tank full of it within weeks. You can try some smaller rainbowfish. Like the gertrudae or furcata.
Beautiful fish, minimum temps seem to be 21c which may just be pushing it a little? Would be fine for most of the year but atm it's a little colder.