Theoretically, you could put them together. Just make sure that the day-olds are actually at least a full day old, and they are standing without wobbling, awake and curious to check out surroundings, etc. In a brooder that big (and that's pretty big), the little ones will be able to get away from the bigger ones easily. There will be pecking, but mostly it will just be a "what are you?" type of peck. It won't be aggression just yet.
I actually just added a "new" hatch to a brooder yesterday, and it already had a chick that turned four weeks old today. There was a little bit of checking each other out, but otherwise they were fine.
Although the brooder ALSO contains many other chicks, and almost none of which share the same hatch date with each other. So he's probably used to little ones being introduced all the time, and just became familiar with what they are already. But pretty much all of them - including the one I just added - were too interested in being fed again to care about the newest member. And NOT fighting over food is probably the most important part of successfully acclimating them to each other.
BUT... each and every chicken is different, so I would definitely be prepared to sit there for a good 20 minutes or so, and see how they interact.