Leghorns are pretty flighty and standoffish, in my experience. What I'd worry about is the fact that the color differences are so dramatic- chickens are very visual and will attack one who's "different".
Birds of a feather flock together.......
You'd need to do a really slow introduction period, after the quarantine and meanwhile your remaining Leghorn would be alone? Do you have more chickens? Might be really tough on her. I'm not saying it can't be done, you'll just have to be very patient!
I don't know a farm raised RIR is going to be any cuddlier either. I have 2, and while they will hang around me, they're not lap sitters.
I wish you were closer, I have some snuggly Ameraucanas I'd give you!
That "birds of a feather flock together" has been true in my experience.
I've always wanted a diverse colorful flock. I do notice the girls who are sisters-same breed, tend to pair up and hang out together...naturally, nothing anyone has done or encouraged. They just buddyup that way. Interesting.