The dimesnions for a typical 20high are (in inches) 24x12x16. Convicts get to be 7 inches long (male) or 4.5 inches long (female). This would be incredibly cruel in my mind. There would barely be room for the fish to turn around in that tank. If it had to be done temporarily, it might be ok. But, in such a tank, the fish will only survive, and really, the goal of successful fishkeeping is for the fish to thrive. In order to thrive, they need adequate housing, and this tank does not fall under that criteria in my opinion.
By the way, green terrors grow to be over 12 inches long, both males and females. The general rule of thumb is that the width of the tank need to be around twice as big as the fish so that there is room to turn around, and the length needs to be at least 5 times as long as the fish, at the barest of minumums. 7 times as long as the fish is much, much better. The fish need room to stretch their fins, so to speak. So, we are talking a tank that is at least 2 feet by 5 feet - like around 120 gallons. 180 would be far, far better.
There are several sites on the web, including the fish index on this forum, that list the typical lengths of the fish. This way you can calculate for yourself what the minimum size tank has to be to keep that fish for yourself, and then worry about compatibility with the other fish you want to keep. Again, fish can survive in a smaller tank, but the real goal is for the fish to live out a full, happy, healthy life. In other words not just survive, but thrive.