Think it depends how trainable your dog is. My working cocker does chase birds, including other people's hens if she got the chance - but quickly learnt she had to leave ours alone - but she is very biddable (rescue dog - can't take credit). In fact, our chickens get quite mardy with her if she innocently goes near their run, and she never responds. But you know your own dog best - go with your gut feel. Maybe you could make sure you could return the hens if there were unreconcilable differences? And I think they're fine in their run if it's a decent size.
That's it really. We have a small garden and a dog, who is a hound and does chase things. I would love a couple of ex battery hens, but would the dog traumatise them? I would expect that she would be very excited and barky for the first day or so, then would get bored of them and ignore them, although I don't know for sure. Does anyone have experience? She is a petit bassett griffon vendeen, just in case you were wondering.
Also, is it cruel to keep them in the run and never let them loose around the garden? I have read that as long as they have a certain amount of space per hen it is ok, but I would only want them if they could be happy.