I agree on the Honey Gourami. The dwarf unfortunately is a fish that to be honest one should not even consider these days. There is the health issue, but there is also the compatibility side, and the honey is much more accomodating for a gourami.
As noted in our profile of the Honey, you can have a small group in a planted 20g tank. Perhaps a male and 2 females, or two males and three females.
But I would not keep the Black Widow tetra with any gourami. This species is known to nip fins and in a large group (8 or more) in a much larger tank than a 20g, it might work, but not in a 20g with a sedate fish like a gourami which is too much of a temptation. If you can return it, or give it to someone, have a look at the Black Phantom Tetra. This is a gem, with similar blackish colouration, and a group of 7 would be nice.
The other tetra I can't say without knowing the species (any chance you could post a photo?).
Tetra in general can be risky with sedate fish; characins have teeth, and they are fond of using them.