Best fish

Blue or yellow German Rams.

For a planted tank you'll need adequate lighting, a fine substrate, and moderate filtration. CO2 injection might be something you'd look into, as well as fertilizers. The Rams would fit right into a 20 gallon planted tank as well.

HRPs. Very hardy, beautiful and breed like crazy. Males aren't too bad on females, but they are cichlids. They will not eat plant either.

I have always wanted to try pygmy sunfish. They do well in planted aquariums.

Jewels would be great!

I doubt it is big enough for jewels. Convicts is pushing it but they seem to not give even a single care as to circumstances. Much like us! I might try keeping some convicts on an old mattress, they will succeed on anything hah!

Seriously though, Rams and some apistogramma are about the best you can do outside of derp livebearers and tetras in a tank the size of my water bottle.

Jewels are fine in a 20g if you have a mated pair by themselves. My pair is in a 20g by themselves because they are hard on other fish, but they never pick on each other.