They should be full size before you attempt to put them with adults.
First expose them to the others by putting them in an adjacent area or pen where they can see each other but not touch.. Leave them there for a week or so.
When you're ready to try to introduce them, let the newbies out to free range for a while...maybe even a few times a day so they know their outdoor space and where to hide if needed. Then let the established birds out and keep an eye on them. There will be fighting. Be sure the newbies have a place to hide. I've used a piece of plywood up against a wall so they can squeeze in there to hide. The older ones usually just want to chase them off. It's the newbies job to run off and get out of the way.
It will go on like that for a while until they integrate. I found that once the rooster started to mate with the newbies he then protected them from the meanie hens.
Another thing you can do to get them to roost together is to place the newbies on the roost after dark. I did that with a single hen I was trying to integrate and had so-so results. They still picked on her but she survived.
It's much easier to integrate several new birds than just one.