I have a strain of platies- blue and orange- that I've ended up with by breeding several generations of them. Some of the older females are about 2-1/2" long and are really fat- when they get bigger their proportions are completely different than when they're little in the store!
I've got way too many of them in a 3' long tank (50? more? I'm going to haul a lot to the next auction I go to).
It's interesting to see how they behave when you've got a bunch in a big tank. They don't really school, but most of them will decide that they should all go somewhere else all at once. They'll all be picking through the val on the bottom right of the tank, then rush over to the right side top, stay there for a while, then go somewhere else. The first time they were all grouped together on the bottom I wondered what was wrong with them!
I've got a similar number of swordtails in a 55, and the only time they're all together is when they're gobbling food. Both types go into a feeding frenzy when food hits the tanks.
I've gone through the same thing several times with trying to get livebearers cheap. Since I end up breeding them, I decided that for me it's just not worth it. A lot of store-bought livebearers don't seem to be raised seperatly from other strains and they don't breed true. I find the odds are better going to a club auction- and if you wait till the end you can get a whole bag of great fish for a buck or two!