I put my new chicks in the brooder with my older one. I put some hardware screen between them so they could see each other but not get to each other. Did this for a couple of weeks then removed the screen so they could interact with each other. Worked out great with a minimum of fuss once they mixed.
Congrats on the new babies! I was stricken with chicken math and now I'm having a hard time integating them together. The 3 older ones all go after the babies and will peck them. I have the babies in a wire dog kennel in the coop. Pretty soon they will grow out of there.
alll 3 groups can see each other. We have 6 older chicks (grew up in one brooder but split to two when they got too big for one) and the little ones in a wire rabbit cage. Im nevous to let the babies with the bigger ones since the big ones are like 4 times the size of my babys. I have welshire babies, and barred rock and some yellow big chicks.