I agree with Mahonri. At 20 weeks old, you should be able to manage it.
I would do more than just house them side by side. I'd throw scratch or some type of food they like on the ground right next to them where the two groups eat side by side but cannot get to each other. After a couple of days of this, I would put them together. There will be some pecking and maybe fighting. It is necessary for them to determine a pecking order. They are social animals and they have to determine who gets the prime territory, who gets to eat first, such as that. You can expect some chasing, pecking, and possibly a little fighting, but your older girls will quickly dominate the younger ones. It helps a whole lot if the younger ones have some room to run away from the aggressive ones. I'd also provide a couple of different places for them to eat and drink. The dominant hens will sometimes get territorial about the food and water. Giving the less dominant ones a separate place to eat and drink can reduce the confrontation.
I would not worry about a little fighting. You will have to use your own judgment since you will be there watching, but the general rule is that if no blood is drawn, let them work it out. If you see one hen that is being over aggressive, remove her for a couple of days. Leave her isolated away from the flock. She will drop some in the pecking order and be much less likely to cause trouble when you put her back in.
If possible, integrate them on the young chicken's territory or in a neutral area and not the established older hens territory.
Now, with all this said, in your specific circumstances, I'd give a lot of consideration to separate feeding and watering areas and then let them work it out. From your posts, it does not sound like it is really all that violent.
Hope this helps. Good luck!