Answer to first question:
I would say that your tank has the stocking of a 20 gallon (right now, at least). Perhaps get another convict and green terror, and perhaps with a few other fish (with good filtration and a bunch of live plants, of course).
Answer to second question:
As soon as I saw your picture, I actually laughed a bit. In my sincere opinion, your tank is much -- MUCH too crowded. Lay back on the plastic plants a bit, maybe take out a third or so of the plants.
Answer to third question:
Listen -- a Convict Cichlid
regularly grows up to a size OF ONLY 3-4 inches. Your convict may not be growing anymore, or it is slowly growing into its adult size. Give your convict a year or so, and it should be half an inch or so longer. Your green terror, I cannot explain its size dilemma.