You could try a number of things. First I would make sure there is plenty of hiding places. Since it's a high tank make sure and get some plants or something that stretches to the top of the aquarium. My female convict spent her first two days hiding in the plants at the top of my aquarium while her tankmates got used to her presence.
Second, yes, add some dithers to your tank. Dithers give your convicts something to chase around and take out their aggression on. Usually for dithers people use more peaceful fish that are fast enough to get away in a chase like some tetras or small barbs.
You can also try rearranging some of your decor to mix up your cichlids "territories" a bit. Although since you only have a 20 gallon there isn't going to be much room no matter what you do.
The less dominant one will probably grow slower due to the fact that she won't be getting as much food as your larger convict.