Delay in the shipment of my 15 chick order, so I purchased some of same breeds from local feed store supplier. By chance, they had one White Leghorn - 6 weeks old, - one Rhode Island red 4 weeks, another RIR at 3 weeks, one Kuckoo Maran at two weeks, and another KKM at 5 days. Obviously I have one juvenile, a couple of grade schooler's and a couple of babies. All seem to get along fine, so far. What are the concerns of mixing ages like this in a brooder?