I put my mottled Cochin bantams with the others at 12 weeks, but with areas for them to hide or get away if chased, and it DOES happen. It's best to put them with the others when they are all a similar size. If they're tiny compared to the others, I'd wait until 12 weeks. And then you'll have to watch carefully to be sure they don't get beaten up. I have mine in adjoining runs, and I have a hole cut in the chain link between the runs. I close it up at feeding times so the big ones don't go over and eat all the little's food. Also separate water, so they aren't chased away and not get enough to drink.
