I don't think any one has advocated a single fish in a tank, they have just said a single vieja sp or a proven pair, there lots of other fish that could go in that tank with them, swordtails, fm,sd,salvini to name a few but IMO I don't think vieja community tanks work unless your talking around the 600 gal mark. I also would prefer to see a single large ca cichlid in a tank then lots of large ca cichlids fighting to survive each and every day
Being isolated for your entire life is possibly the worst torture imaginable. Fighting to survive each and every day?! lol