There's a difference between "recommended size for breeding" and "smallest possible to keep them alive". I didn't have much luck in a 10 gallon. In my air conditioned shrimp room the pH and hardness would swing just from evap and topoffs, they would get berri d but then die or give birth and the babies die.
And I also don't use a 20 long, the evap is extreme, and light is too intense. I use a regular 20 gallon tank, and they do very well.