Will there be a week and a half difference when you get them? I would keep them separate until the new ones have their legs under them, and can zip around in the brooder. What worked well for me with a 10 day spread last year was to put them in the same brooder with a wire divider for a few days, then I'd put one or 2 older ones in with the younger ones for supervised play times. I gave them treats at that time. I continued until all of the older ones had been with the littles for a bit. The next day, I pulled the divider for the day, but put it back at night, then the following day, I took it out for good. I probably was over cautious, but wanted to be sure there were no problems. Your biggest concern would be that if the chicks got cold, they'd pig-pile and the youngest ones could be killed.