Its not true of dogs either (the growing up together thing). Many times when the animal gets sexually mature, and EVEN it it gets neutered early, fights can start at any time. Vicious fights. I've had it happen, and its not pretty. So I can see the same thing happening with fish. Babies get along--baby fish, cats, dogs, horses, birds, etc. When they get the hormones going though, their whole outlook can change, even if there's not a mate anywhere in sight.
I am one of those who also doesn't have a good LFS within an hour's drive away. I will not, under no circumstances, buy from Petsmart or Petco again. In nearly every tank, you will see fish gasping at the top, sitting on the bottom, or with clamped fins. Ich is sometimes seen, and its so obvious that there's no way the employees haven't seen it. The only fish I will buy at either of those stores are bettas, as they are kept separate from the central system (in bowls, of course) and so are spared being exposed to what all the other fish are. They can still get sick, but if you quarantine them properly, the chances are somewhat lower. I know I shouldn't buy bettas, as it only encourages the store to carry them, but now and then I just cannot push myself to walk past them without "just looking." That, of course, leads to the inevitable buy. Those bettas practically jump on my back and hang on so I'll get them out of there! I do have a 55 that is not fully stocked, so I've been known (more than a few times) to get a female betta here and there. One that looks healthy, and is active. But as for platies, mollies and guppies especially, most seem to be of inferior stock that are weak and destined for disease, if not already diseased. Every single guppy or molly I've gotten from either of those places had passed away within days or weeks of purchase. The LFS that is an hour away is a lot better as far as diseases go, but they have only a 48 hour guarantee! Petco has a 30 day guarantee, and Petsmart has a 14 day guarantee, I believe. But this still would never sell me, as I get the fish to enjoy them, and give them a happy home, not watch them die slowly. The money is secondary.
Enhanced glow fish 
