Oh, in the other thread I thought you said the tank was 24 inches long. Having looked at the pics, it looks like a cube, not a column.
That's a decent footprint.
In regards to the festivums, have you tried rearranging the tank? With cichlids, and other territorial fish, it can help to move things around in the tank. As it stands, the new cichlid is encroaching on the old ones territory. By moving stuff around, you break those old boundary lines and make the fish all carve out new ones, giving the new one a chance to establish itself. If you want to take it one step further you could remove them and add them back once the tank is redone.