I know this thread started a year ago, but it looks like it is going again, so I would like to add.
I currently have a 250 gal Vieja/Paratheraps tank going. It currently houses argentea, black belts, regani, zonatus, fenestratus, bifasciatas, with a pair of nics. There is definitely some aggression as to the pecking order, but nothing unusual. The fish average 4" to 6". I know that they still haven't reached their full maturity, but they all look great so far. As for temperment, the argentea and the blackbelts seem to run the asylum. The regani is far less aggressive, and the bifasciatus are absolute pussy cats. They are big, colorful, and mellow.
I also have a mating pair of Zonatus by them selves. The male is 10.5" and the female 7". They are awesome. The male has never (knock on wood) been aggressive towards her. They are wonderful parents. I just took a video of them today, if you all would like to see it, let me know.
As for the Heterospila. I agree, cant find them anywhere. I did get one donated to me from a friend. It is about 11", and very beautiful, but cant find anymore.
So I guess as aggression goes, its a crap shoot. I have people stop by all the time and are absolutely amazed at the fish that I have coexisting in the same tanks. I dont know if it is luck or what, but I have know complaints about any of the Vieja or the Paratheraps.