It is normal in my flock for the mature to bully the immature if their personal space is invaded. Until my pullets reach a certain level of maturity they are likely to get pecked if they get too close to the adults. My immature chickens form a sub-flock. I have enough room that they can avoid the adults during the day and they do not sleep on the roosts with the adults at night. When the pullets start to lay eggs they are generally accepted by the adults, but until then the pullets quickly learn to not get close to the adults until they are that mature. Otherwise they may get pecked.
I use the same basic method you did, housing them side by side for a while. When I turn them loose to roam with the flock there aren't any issues. But I have a lot more room than you do so they can stay away.
That doesn't mean it is hopeless. It is very possible they will be able to coexist until the pullets mature enough to join the adults. I'd keep trying to let them mix, but give them as much room as you can. Don't try to forced them to be close together. Put clutter in there. That means give them places they can hide under, behind, or over to get out of sight of the adults. But don't make dead ends, give them a way to escape if a hen chases them. Leaning a sheet of plywood against the fence is common. Give them perches high enough they can not be pecked by the ones on the ground. Maybe set something on cinder blocks they can hide under. Have multiple food and water stations so they can eat and drink without being beat up, the adults can be bullies in protecting the food or water if it is not spread out.
It's not always as rough as I may have made it sound. People do this all the time. It can be a nervous time.
Good luck!