Introducing new pullets to older chickens. When? How?

[AKnewbie]

We have 5 Australorp hens that are over a year old right now. We started with nine. And gave 4 to friends. The 4 we gave away seemed to be the most henpecked ones (feathers missing on the head). We're babysitting 2 of the 4 that we gave away and as our friends dropped off the 2, there was immediately a scuffle between the chickens that stayed and them. One or two of our remaining ones just tangled with the 2 that we are babysitting.

Currently, we also have 2 RIR's that are around 3 weeks old. I had initially thought that they would just integrate within the flock that I already have but after seeing the scuffle between our chickens and the 2 that we're babysitting, I am worried. Note that the 2 that we're babysitting were part of their flock until about 4 weeks ago.

Any tips?

 

[five is my dog]

when they are about the same size.