Ok here's the thing, and a lot of people don't know this or don't experience it because of the way they keep them. In the wild, thorichthys often times maintain colonies where multiple pairs will be in the same large territory. Granted each individual pair will have its own breeding area but the pairs get along well enough together that often times they'll even help guard each others' fry. They will also work together to defend the territory of the colony should a larger predatory fish come along as opposed to an individual pair guarding its own little spawning area.
If you only keep them in 55 or 75 gallon tanks you are less likely to see this happen, though I do think you could at least try to keep multiple pairs in a 75 gallon. I would get about 12 juveniles and grow them up together, setup the tank decor just right and see how they get along as they get older. Oh and add about 15-20 or so platys or swordtails.
Now if you were to put a large group into say a 125 or even a 180 gallon you are much more likely to see this behavior. You could even in that size tank keep a larger open water fish with them so long as it's not too aggressive.
