The brooder may be a bit cramped for the new ones since the bantams are growing so fast. I would introduce all of them at the same time to the larger brooder with fresh bedding. Also add a second waterer and feeder at some point. That will prevent the younger ones from being kept from water and...
Three weeks old is about the most I would would wait to mix chicks. It is better that the older chicks are bantams. I have raised bantams and large breeds together from the start, and then I added a few more about 10 days in. They all did well. Bantams are bit more fragile for the first week...