Howdy neverlaysup and Welcome to BYC
Just some of my thoughts for you but others with more experience may have other ideas.
I have found time and patience to be the best method.
Like you, I have the ability to separate my youngsters from the adults and I personally like to keep it that way until they are at least 3 months old; just to give them the opportunity to fill out a bit more before they have to take on the big gals The runs are joined by wire so there can be lots of looking and vocal interaction but no touching.
As space is the best tool with integration, I then let them have supervised free range time but pop them back into their divided runs at lock up time.
In the beginning, if the big gals are free ranging, I close them out of the run, remove the divider and let the youngsters explore the areas they have not been in; this gives them the opportunity to become accustomed and learn where to run and hide if need be. An area they have never been in plus big chickens can be traumatic for youngsters so they will benefit from being familiar with the surroundings.
I also switch this around and let the youngsters out for a free range and keep the big gals locked in; giving the youngsters the same opportunity to get used to the garden, hiding places etc without the big gals giving them grief. [Note: the big gals are never impressed when this happens and have no qualms vocalizing their disapproval ]
If you have an existing hen who is fairly easy going and placid, you could try introducing her to the youngsters and if she bonds, she may be able to help introduce them to the flock. If you have any bullies, I would definitely keep a close eye on them.
Distractions are also a good thing to have around so the older girls have something better to do than chase and peck the young ones. If they are in the run together, maybe some leaves or clods of dirt for them to scratch in or hang a head of cabbage for them to peck at. If you have the room, some hiding places for the youngsters or extra branches etc will be helpful.
When they are integrated in the run, I also provide two lots of food and water for a couple of months to ensure that all have access.
You can feed the whole flock pullet grower if you have a dish of oyster shell on hand for your older hens who feel the need for some extra calcium; the youngsters will not eat the oyster shell as they do not need it yet. When the pullets reach laying age you can switch everyone back to layer feed.
There are some good tips on this BYC thread also:
Good luck with them and please let us know how you go?
Edited by Teila - 10/4/15 at 3:31pm