I think we understand what you want, but you just don't have the right equipment right now.
Juvenile Green terrors grow pretty slowly and they will not 'pair up' at 2 inches. They will need lots of time to grow and mature before they are sexually mature. I'd say close to a year if they are only 2 inches. I have 5 right now in a 75g, and the biggest is about 5 inches. Still no sign of 'pairing up'.
Although jack dempseys probably mature faster, they will need to be atleast 4-5 inches before they pair up. That means you have to keep all of them until they get atleast that size and that's just not going to happen in a 20g.
Be patient. You will be more satisfied in the long run if you do this the right way.
All that being said, I think either would be a good choice. Dempseys and Terrors are both awesome fish.