Hi, Alan
Congratulations on the ex-battery hens. It's very unlikley that they are RIR - it would be very unusual for a pure breed to be used in a battery system. They are more likely to be a hybrid specifically bred for intensive systems.
If you introduce a new hen, there will inevitably be some scrapping until the pecking order is extablished - who turns out to be top hen, who knows. I think it's more likely that that a single, young hen going into an established group of three is going to get a hard time. If you go ahead, it will be easier if you can keep them seperate but able to see each other for a week or so - we corn our together through the wire so they get used to seeing each other. Mix them for the first time at night after they have gone to roost, ideally in new surroundings so everyone is in a new place.
We give our hens free access to mixed grit in a hopper and let them help themselves. If you change to pellets, do it gradually. We did use mash but it was so wasteful we changed to pellets. In a less intensive system, pellets are fine.
Hope this helps,
Rosemary