I couldn't find much on this topic so I thought I would post my results. I combined two day old chicks with three week old chicks in the same brooder in my house. The 2 day old chicks seem to take to the older ones as their mothers and it seems to have worked out pretty well. The older birds picked at the young ones beaks a couple times but that didn't last. Everyone seems to be very happy! I just thought I would post this in case anyone wondered "what if..."