I just got the Chicken Health Handbook, which is a very good book. One thing that worries me is that it says: --Do not mix older chickens with young ones --Do not add new chickens from different sources I have 2 pullets (8 months old) and I want to get more. I would be getting young birds from a different source, but one I think is reputable. Now I am worried that I am risking my hens health. Is it really that bad to add to your flock?