New Chix picking and pecking

We have four brand new babies, our first chicks (6 days old). Everything was fine until this evening when our Easter Egger (the smallest) has decided to start picking on our Barred Rock who is the largest chick. Every time she pecks at her in the brooder there is terrible screeching and the the BR tries to run away but crashes repeatedly into the walls.
Are they just trying to establish pecking order? Is this a concern and do we need to separate them or will they work it out?
Wer are currently brooding in a large rubbermaid with red light.

Thanks for any advice...

 

Thanks Judy,
We took them outside for a "run" today as it was nice and warm, but its been very cold here at night. They are much too little to be outside the brooder at night. Ideas??