If the temps hold true, I think both groups can go out at the same time. The size difference shouldn't be an issue, and the younger ones should be okay at those temps. If you want you can offer them the option of a heat plate but they may not use it at all.Yes, so I was thinking in about 3 weeks. We should have the coop ready by then. Google says avg. temps where I am are in June are 60 - 82. The bigger girls will be over 6 weeks and younger a little over 3.