Well I think you'll get a difference in oppinion on this. But I never introduce until they are about the same size as the other chickens. If you do and you noticed this, you could have dead chicks. Chickens can be very mean at times and they don't like new comers and it takes a while to get everyone adjusted to the idea and the chicks need to be big enough to defend themselves. I like to have seperate pens, fencing inbetween is always good idea and then they can see eachother and get use to each other. I have 22, 12 week olds I won't be introducing probably for another month or so. I like to let them free range a bit first and see how they all get along before I would ever consider putting them all together. Good luck!