When they begin to lay, they will generally get a boost of confidence.
What you need is a way for the new birds to 'bow to the queen'. You said you had swing and perches, but I am wondering if every bird can see every other bird 100% of the time. You might need hideouts, where a bird can step out of sight. Where a feed bowl can be set that while eating at it, the bird is out of sight of the birds at another feed bowl.
When two birds meet, one need to bow to the upper bird, but stepping away. Sometimes what happens in smaller flocks, is the younger does not fight back, but there is no where for her to get out of sight, so the upper one thinks she is showing no respect, and attacks again and again, harder and harder.
I would also try adding two young to the one older, with the other older where you had the younger ones. But first I would add a lot more clutter, which will slow down birds, and let birds get out of the way of the attack.
Another thing that works well in small flocks is pin-less peepers. Put them on the old biddies for 2-3 weeks, and by then, the younger ones will be more confident.
Mrs K