Dwarf, croaking, and honey gouramis do well in 20 gal tanks. Since your blue gourami is a female, you probably don't have to worry about aggression. If the blue gourami was a male, I would advise not adding any more gouramis to the tank, unless your tank was well planted (lots of natural floating plants, etc.). Tetras are very peaceful and generally mix well with any fish, and they'd do well in a 20 gal tank. Avoid barbs and other fin nippers as they'll go after the little feelers on the gourami. Most gouramis are territorial by nature, so plants, driftwood, and rocks are definitely worth considering.
