Get a colony of zebra plecos for your 20 g. They won't get big , look cool and will wallop your wallet as you apparently want to do.
Ohh a biotope would be very cool, a sandy bottom, lots of smooth river stones and few planted areas.
I find rare fw are few because most fw fish are bred in captivity. If you go onto a saltwater forum and ask for rare SW fish you will get tons of responses. Most of them are deep water reef fish.
So yeah I think the OP is looking for the fw equivalent of Bells Flasher Wrasse, Bandit angels, masked angels, dr Seuss fish, conspicuous angelfish, candy basslet, coral sea mandarin wrasse etc.