Put the bantams in a large cage in the coop where the others can get used to seeing them. Leave them there a few days. Introduce them to the flock only if there is a large area. The larger the better. If any of the big chickens are overly aggressive, catch them and put them in the cage. Leave them there a few days. Chickens have short memories, and then when the bullies come back, they will be the new kids. This will only work if your flock is small and you have adequate room. Also, if the Bantams are new, make sure they are quarantined adequately before they go near your coop. If any of of your chickens are the aggressive type, you might not be able to mix them.