Money, you cannot change the behaviourial blueprint of fish. Any more than you can get a grizzly bear cub and when it is full grown assume that it will never revert to its natural behaviour just because it has grown up near you. Animals have their basic instincts and we can't change them.
An example from my own aquaria: I got a group of 9 Aphyocharax paraguayensis one time, and put them in my 90g planted tank. They are small fish, 4.5 cm max, described in Baensch as "peaceful community fish." I knew from other sources that they are active and a bit boistrous, and sometimes nippy, but they would be with all smallish characins, no angels or long-fins to worry about, and in a larger group such tendancies do sometimes wane a bit (e.g., this is said of tiger barbs).
Well, the second morning I went to feed the fish, and all I could find in a 90g tank of some 85 fish were these nine tetras. All the other fish were cowering amongst the plants. I put in the flake, some of the others came out to eat, and got chased back and wouldn't venture out again. No biting, just "playful" chasing. After a day and a half of seeing nothing in my tank but 9 tiny tetras, they came out. Within hours the other fish began to venture out, a bit pale in colour, and it took a couple more days for them to regain their confidence that the "terrors" were really gone. Then ich broke out, brought on not by the new fish, but by the stress I had unwittingly thrust upon them.
Fish when young are still growing, just like people, and may not exhibit their true colours, so to speak. And some people have this fish or that fish and it behaves contrary to the norm, but this is the exception--and you never know when it will not revert to its natural tendancy. On another recent thread, someone had angels and (I think) some tetras in a tank, and added a group of cardinals. Within hours the cardinals started disappearing--into the angels' mouths. It's their natural instinct. And like we have been discussing in other threads, the safest way to ensure the health and well-being of your fish is to provide the most natural environment. Don't risk their health by introducing tankmates that have the distinct possibility of being problematical.
Byron.