Just one brichardi? With no breeding aggression and if your parameters are not too extreme I think most top dwelling livebearers might work. (no personal experience with this) But depends what exactly you are trying to achieve.
Neolamprologus brichardi do no really need any other dither fishes. They do like more of themselves though. With a 20 gal, 3-4 is about max. Try to get only one male and a few females. 2 males may fight. They are super easy to breed too.
PS--I do not know how to tell them apart.
Hi I was only able to get one brichardi from a fish store as I traded a jewel cichlid for it. But I don't want to get more because my lfs sells them for $35. That's why I was thinking of what other fish I could add. Do you think I could add 4-6 danios in the 20g?