I have 14 barred rock cockerels that came from Ideal Hatchery as extra "males for warmth". As they grew, I had one who was much smaller than the rest, and was even smaller than the bantam cochins they came with. I kept him in with the bantams, so I could watch him and make sure he got plenty to eat. He feathered out very slowly. By the time they were 8 weeks and I moved them to their outdoor home, there was another about the same size as the runt.
Today they are 16 weeks old, and the cockerels all vary in size a bit, but I can no longer tell my two little runt boys from the rest.
Just keep an eye on your babies, make sure they have equal access to feed and water, and they should be fine. It's interesting to watch the different growth rates of all the babies, isn't it!