The OP gives some details in this thread. Basically two more mature pullets and a cockerel are not accepting more immature pullets as equal members of the flock. One pullet especially is attacking them. The three older ones are 18 weeks old, the three younger pullets are 14 weeks.
https://www.backyardchickens.com/threads/flock-not-integrating.1330928/
From my observations it is normal for mature chickens to not accept immature chickens as equals in the pecking order. If the older ones are still immature they can sometimes be even more brutal than truly mature hens but you can still have problems with adults. Typically the younger quickly learn to avoid the older as much as they can. If they have enough room to do that things normally go pretty smoothly, but there can be exceptions. Some chickens are just brutes. Your older pullet may be one of those. Hopefully she will change as she matures.
You say they hide under bushes when they all free range. How much room do they have when they free range? A photo might help with that. I'm not surprised there can be issues when they are in the coop and coop/run but with more space it sounds like you have an over-aggressive pullet. Then you say when you remove that pullet the cockerel becomes aggressive. Is that when they are free ranging or when they are in the coop/run? Isolating that pullet is a good idea but I don't understand why the cockerel then becomes aggressive toward them. Is he trying to mate with them or beat them up?
Typically my pullets are accepted into the flock about the time they start to lay. Until then they avoid the older chickens during the day and will usually not sleep on the main roosts with them at night. They are afraid of the older chickens for a reason. If they invade an older chicken's personal space during the day or on the roosts there is a reasonable chance they will get pecked. When I open my pop door in the morning it's pretty normal for the younger pullets and cockerels to be on the roosts while the adults are on the coop floor. And when I have different ages of immature chickens the youngest are generally hiding under my nests, it's a good hiding place. In both cases, the less immature are avoiding the more mature.
So what do you do? Find some way to get to the point where the younger pullets are laying so no one is injured or killed. That should solve your issues. How you get there is the question. Often when you remove the aggressor the rest get along but you say that is not happening with your cockerel. If he is trying to mate with them that's about dominance, it's not aggression. He's not likely to hurt them. Your small numbers may be contributing to that behavior. You can let it go but observe to see if any are actually getting injured.
Dobie has some good suggestions about multiple feeding/watering stations and places to hide under, behind, or above. Often integration just isn't that big of a deal, but sometimes it can be rough, even when you have lots of room and do everything right. Each chicken has its own personality and sometimes there are personality clashes.
Good luck!
