I give the new flock and the old flock a little time together each day, increasing it little by little each day, depending on how peacefully they are getting along. Have the new ones sleep in a separate area, so there won't be any bullying in the morning before you are there to supervise. I do not keep them together round the clock until the new ones are adult size, and by that time the young and old ones are well matched and used to each other. You might be able to merge them into one flock sooner than that, if your new ones are standard sized. They might be as big as the banties by the time they are 6 wks old, or so.