If it has already established the entire tank as its territory, it may be difficult to add anything else, especially any other cichlid.
If I were to try another cichlid, it would be one that looks entirely different, different mouth shape, different body shape, etc etc, because anything that looks any bit similar (predatory mouth, torpedo shape) would probably be seen as competition.
And most dither fish, would be thought of as dinner., unless much larger than a mouthful, and then compete for the limited space.
I have never been ablate keep any tank mates with motaguense in that size tank, except a compatible one of the opposite sex.