You can keep larger dither fish like Columbian tetras (10+) with a green terror. I personally wouldn't keep other cichlids as it tends to destroy the male's nice fins, either being nipped or ripped as he chases the other cichlid.
I agree with this, especially because you have plants. With another cichlid you would probably run into territoriality issues, which often results in plants being torn up to aid in cichlids need to keep tabs on each other.
Green terrors are from west of the Andes where waters are hard, and the cichlid family is less biodiverse, than the Andes eastern side.
If I was set on another cichlid, and willing to take the chance of having my planted aquascape torn up, one of the Red Hump Geo's (G steindachneri. or pellegrini) might be my choice. They look dissimilar enough that they "may not" be taken as competition, like many more similar looking other cichlids would. Or a small shoal of G.T.s, 4 to 8, heavy on the females, all grown up together from juvies.
I keep a close cousin to your G.T.s, about a dozen Andinoacara coerleopuncttus in a heavily planted 180, without any other cichlids as tank mates, there a a couple tetras, a pleco, and a goby,


and to my eyes, fill out the tank quite nicely and with little to no territorial aggression, or redecoration of the plants.
