OK, so I'll have some time to think after I start the cycling; but I'm going to go with the 20 gallon. 23.75" x 12.625" x 19.75".
Based on the advice, I'll avoid Betta/GBR combo, so how about GBR and Gourami (maybe a dwarf one)?
I keep finding mixed messages as to how many of GBR and Gourami...