Welcome to the forum from Louisiana, glad you joined.
It sounds like you don't have much room, that can make integration harder. Another thing working against you is their maturity levels. Until my pullets start to lay they usually form a sub-flock and avoid the adults. If they invade the personal space of a more mature chicken they are likely to get pecked. That's why they are so afraid of that older hen. That behavior is very normal in my flock.
So what can you do? A lot of that depends o what you have to work with. How big in feet or meters is your coop? Your run? How are they connected? Photos inside and out could be very helpful.
There are some generic things that we recommend. If you can house them across wire for a while (a week maybe?) so they get used to seeing each other can help. Give them as much room as you can. You can improve the quality of what room you have by adding clutter. Clutter is things they can hide under, behind, or over that breaks line of sight or puts them out of reach. Having widely separated food stations and water stations can help, especially if they are protected from line-of-sight. Without knowing what you have to work with I can't get any more specific than that.
Good luck and once again,