I would agree that its all about tank size.
But you seldom see 2 Vieja species in nature living in the same place sympatrically.
I believe this is because they both would eat the same type foods, use the same territory etc etc."compete" Putting together cichlids of a different genus, sometimes works better, because some of the competition factors are lessened if the tank is large enough. This may be because putting a general carnivore like Parachromis with a vegetarian like Vieja may create less friction, at least when not in a spawning mode.
You might say, well an aquarium is not nature, and I agree, far from it.
In nature a Vieja might have 10,000 or 1 million gallons for a group of each species to share, so if a human dropped a syn in an area populated with maculacauda, it might actually work(?),
but in a tank that is more the size of a small ditch or tire track in the mudd (such as a 125 or even a 220 is) its pushing territorial limits.
And Black belts are some of the most successful and wide spread of their genus in Central America, which I believe may be because they are very, very, aggressive as adults in defending feeding areas, and spawning territories..
