Is this a good idea?

Not really. You take the chance that your older pullets could possibly kill your younger ones.

 

How many chicks do you have? It's usually a good idea to wait until the chicks are nearly full sized before introducing them to the big girls, but you can probably cheat a little bit if you only have two grown hens and the chicks outnumber them. You'll still have to keep a close eye on them though.

 

Well....I would be willing to chance it. Look....First of all we are talking about 2 hens....not roos....and 2 hens from some very calm breeds, known to be great moms. Now we are also talking about multiple chicks that are 12 weeks old...the age many are for slaughtering age. There is safety in #s. And then you can do a Rhoda job.....Introduce them, with you in the coop, sitting on a plastic lawn chair, chilling out, so you can babysit. They have to get used to each other eventually.

 

I would put them side by side for a while at least so they can get used to the sight and sound of each other. 12 weeks is pretty well feathered out and they should be able to out-dodge the bigger girls but I wouldn't just dump them together.

I keep mine in seperate pens where they can hear and see each other and go beak to beak for at least a few weeks, then open the two pens so they can mingle, though each stayed in their own sections for a couple days with the youngsters making only brief forays into the big girls area before they dodged back to where they felt safe... they slept on their old roosts too. Then, I snuck them onto the roost with the big girls one night, and let them all wake up together, and blocked them from their old pen.

The older girls wake up a little huffy, but don't usually get nasty.