pecking order or meaner personality just not sure, please help

I started raising chicks 6 mths ago . I started with 6 ( 1 rooster 5 hens ) . Then I wanted some guineas to help keep snakes, ticks, fleas & etc out of the yard so a few weeks later I got 4 guineas & 3 regular hens they are now 4 mths old. This group of 7 was together from birth. I kept them in a separate area next to the others separated by a fence. I slowly introduced them to the yard to free range with others.
Things went pretty good for a while , just normal pecking & chasing. Recently the guineas have started to peck & run the 3 hens they were raised with off & they have to go to hiding places. They are scared to even go into the coop at night. I have to go find them & put them in there so they will have protection. I feel so sorry for them. I don't understand why since they were raised together they are doing this. The guineas follow the rooster around a lot.
So should I remove the guineas & put them in a different coop or let them continue to do this to the 3 youngest ? Or maybe get a rooster for just the guineas ? I just want to do the right thing so they all will be safe & not stressed.
Thank you for any help you can give me.

 

Thank you all for your info & help. After I received the 1st reply I had decided to separate the guineas from the regular chickens. But the next day I went out & found one of my smaller reg chickens dead . There was no blood or sign of a predator . I do believe it was the guineas, but no proof. So I decided to give them away to a friend who raises chickens & wanted them.
So far things have been going well since they are gone. I also believe the youngest chicks ( 4 mth old ) are less stressed & they also go right into the hen house at night now where before they hid in the bushes.
I didn't think mixing the 2 kinds would be a problem since they were together from birth, but I guess they just were more aggressive. I wish I had never got them , but too late now , I have learned a hard lesson.
Again thank you all.