I recently had to do exactly this. I got 4 chicks (3 weeks old now) and then a week later got 3 more (2 weeks old now). The younger ones outgrew my small brooder so I had to move them to the larger tub with the old chicks. One of the older chicks will sometimes run over and peck at the younger ones for no reason. The little chicks do screech when this happens. Is this something that will work itself out or am I going to need to come up with a different plan? I don’t really have the ability to set up another 125 gallon tub as the one I have now is currently in my dining room. I live in the suburbs so I don’t have a ton of space.
If the pecking is only occasional, and there are no injuries, then it should work out in time. Just watch and see.
If it seems like a bigger problem, maybe put the big ones in the smaller brooder for a few hours, so the little ones can explore the big brooder and get comfortable there. Then put just one big chick back in the big brooder, and a few hours later put the other two back. This spreads out the new things for the little chicks: new place, new companion, other new companions. And it interferes with the big chicks thinking "invaders in my space!"
If the big chicks were very aggressive, you might have to divide the brooder with wire mesh, so the chicks can see each other but not peck, and give them several days to get used to each other, then try again to combine them.
Body language is important for chickens, so letting the little chicks get comfortable in the space means they act differently, and the bigger chicks may respond differently to comfortable chicks vs. uncertain chicks.
But from what you said, I think they will most likely be fine if you just leave them alone. The other ideas are just in case it doesn't work out well.