Now is the perfect time to mix the group. At 2 weeks they are beginning to develop the pecking order, but at this point, they are "not yet into that". LOL So go for it.
You will have to adjust your heat according to the younger ones. In other words, the older ones are now the same age as the younger ones as far as lowering the temp 5 degrees each week til 6 weeks old.
Enjoy your new babies! So glad you could join BYC.