Fishy Friends And Clicks Groups ?

Hi
I was wondering do fish that are bought together at the same time form a bond ?
If you add more of the same species do they get accepted into the old group or does it take time for them to make fishy friends ?
Ally

I have noticed with my penguin tetra and glass bloodfin tetra that there is usually quite a calm environment, but perhaps with one fish chasing another sometimes. I've always thought this was a group heirarchy thing. It usually ends with them all back where they were in the first place, and none of them come to any harm.
The tiger barbs are a different matter. They are very obvious with their dominance, and when I have added more I have chosen younger/smaller barbs in the hope that they will be more submissive, and changed the decor just before adding the new ones. So far it seems that the same one (largest standard tiger barb) is still the boss.
I was previously worried with one of my temperate tanks. I'd had to slpit up my v. platies due to babies. I only had one male, so put him in a different tank. Eventually I added peppered cory and white cloud mountain minnows to his tank. I still rearranged the decor, and everything seems fine in that tank. Sometimes Daddy platy swims with the WCMM, sometimes he hangs around in some of his favourite spots on his own.
It can also be down to the personality of the individual fish rather than the species. Even some generally placid  species can have an individual who can be a bit of a nightmare. Don't forget that this is nature, they are a group of fish and there will usually be a heirarchy rather than a group of friends, even if we like to thing of them as such. 

I think that it's so that the previous fish don't see it as their tank and see the new fish as invaders.
Also, they spend more time exploring than hassling the new fish, and by the time they've finished exploring they've got used to having more fish in the tank.