I would say it's totally doable, especially in a 20 gallon tank.  If you want to breed them, it might be less effective compared to keeping  them in a 5 gallon tank by themselves.  I'd try it and put a hanging mop in there, the gabunense would likely be the only fish interested in spawning in the mop.  The other way you could try it is to have the tank heavily planted with floating plants and the fry may show up.  Good luck and let us know how it works out.

Congrats on the bivitatum. A beautiful and peaceful killi. This species will breed and young will appear in a heavily planted tank. You may want to try it at some point. Just saying.