I'm a little worried about this, too. We bought 3 pullets (can't have Roos in the city). Two of them (Americauna and Light Brahma) are getting very big quickly. The Rhode Island Red isn't growing as fast. The Americauna and Light Brahma are chest bumping, too. Ours are 2 weeks old. The bumping shouldn't be a crowded issue, in our brooder, either. They are in a 2.5' x 4' kennel. The Americauna is roosting on things, too. Her latest is sitting on top of the feeder. I'm really worried I'm going to end up with one lonely hen out of this. I'm hoping the Rhode Island Red is a runt (can chickens have runts?) - or that the other two might actually be a little older than what we were told.
So some people will say no, but I say yes. I had 10 chicks when I first started that were sexed when I got them. 5 roos came home. It was hard but with in a year all but 2 were gone.