At what age is it safe to put my 6 week chicks in with my 4.5 month olds???

[Popcorn Mcgee]

We have two six week old chicks, a wyandot and a polish, and three four and a half month old chickens. The chicks are outgrowing their brooder and have been spending the days outside, but I've been bringing them in at night. Yesterday I put them in the big run with our three older hens and they leave the chicks alone about 70% of the time, but I keep catching my Brahma and Cochin taking turns chasing after the chicks and pecking them. Should I not have put the chicks in with the older hens yet, or is this just the pecking order and if so, will they eventually stop? They spent their first night out in the coop with the older hens and when I went to let them out this morning all seemed well, but then after I made breakfast I checked on them and my Brahma had chased them behind/under a corner hanging waterer. I had to go out and rescue them and then I turned the hens loose to free range so the chicks would get a break. Also, the chicks have been spending the last several days in a pen beside the older hens run so that when I did introduce them they weren't complete strangers. This group is my first try at chickens and I am open to advice. Thanks.

 

[fried green eggs]

I've had good luck by locking the younger & smaller chicks in a wire dog crate in the coop at night for a week and only let them out during day together. When I see the older ones sleeping by younger ones - all goes well when the cage is opened up and removed. Good Luck.