Cockerels at any age should leave if you don't need them. It's better not to have problems with the flock during there growth stage, cause it will happen. The aim is to have a stress free flock.
You can integrate one young hen if your flock has a rooster. I do it all the time since my sex ratio hatches differ each time.
But without one, it will take a very long time for the flock to get use to the new member. So constant monitoring is required to keep the new hen safe. And intervene...