I wouldn't worry about the bigger girls, a cat is no match. My feral cats don't even dare walk across my chickens run, the girls will gang up on the poor cat.Thank you.
We’ve had another set of chicks in the same brooder and it wasn’t an issue but we are definitely going to up the security!
However the same cat just pounced at one of our bigger girls (which she’s never bothered before) and we don’t have a rooster so she may be going to a new home. It’s such a tough situation.