What dimensions is the tank? The volume itself does not really matter, for stocking of territorial fish, it is the length that does.
When I had my green terrors, the pair was given to me in a 3 ft, 55 gallon tank and both fish were stunted. If you are talking about a tank that is 6 ft long by 16 inches front to back by 1 ft tall, then your idea might work, but I'd still want more space front-to-back. In a tank shorter than 6 ft, I would never want to mix two green terrors with anything, as the anything is likely to get killed off.
If your tank is only 3 ft long, then you will be able to have a pair of convicts, and that would be pretty much it from the bigger cichlid list.
Big cichlids need their space, so they really can only be kept well in larger tanks.