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.