My experience points me in exactly the opposite direction. I have kept a lot of them for much more than a few months. They do a lot better in groups. They are indeed social fish, and school depending on the society. If you get three (mff), it is likely that two will pair and the odd man out will be attacked and worried until it dies. If you get seven, you will never see this happen because it isn't one "odd man out", it is five odd men out. They will all hang together in that situation, not interacting to the level of a pair of clowns, but definitely schooling.
Any paired fish of any species will become very territorial and the pair is likely to do some damage on conspecifics, but if it isn't directed at a single individual, than it will likely not cause any damage.
With a fish that is BARELY sexually dimorphic, you have no way of telling if you have pairs, or a bunch of males. Three males will fight to the death. A male and female will fight until they get down, and then start hassling the odd man out. Perhaps that was your issue Bill?
Wild caught specimens are not going to be related, and the vast majority of these fish being sold are still wild caught. They are often very difficult to get onto frozen prepared foods, so perhaps that was your issue NBR?
Lastly, on the topic of brooding and inbreeding, fish have no restrictive mechanisms to avoid inbreeding other than the odds. The comment on them being from the same brood bears no influence on their potential for breeding or how they will interact. Fish basically hit the water and split. Their is no parental care into sexual maturity in any species.
