Depends totally on the hen with the one chick. By 6 weeks, the lone chick is now a "spoiled brat" and could resent any new chicks, even if the hen accepted them. My experience is that it is not good to add chicks after even a few days old. Others have not had problems--using various methods to integrate into a mixed flock. You are on your own to be creative. I think your present idea may be best----hen back with the flock and integrate just the chicks, but at their age, they might take a bit to get along, but should, in time? There is no one way and it could work or not. GOOD LUCK, chickens are worth our management trials!