I don't know if I would have them "living" in the same pen, but I certainly think they can help with rodents. I have a 4 month old male kitten that I got to be a mouser. I am waiting till he is a little older to let him outside, but I am working on training him. There is a good cat training book called "Cat Training in 10 Minutes a Day" I am going to start working with him with that method and continue to teach him to search out and kill mice/rats meanwhile not harming the chickens. Fortunetly, a chicken was weak and I brought it in the other day and that gave him more opportunity to get up close to one. I have a cat carrier that I will put him in to go into the chicken shed so he gets used to them. I have every bit of hope that he will be a good mouser while being nice to the poultry.
Also, a little adolescent mouse got in the house last night around the time my husband got up for work. The cat was unsure, but my husband encouraged him to go after it. The mouse was clearly dying as it couldn't run. It was the perfect first lesson. My boy ended up so engaged with "playing" with the mouse that it was hard to get it away from him. He wasn't even interested in the treats I gave him as a reward.
We are actively training and working with him. I am waiting on a few more supplies to begin the real clicker training, but to encourage him, I am playing with a cat toy on a string multiple times a day to get him conditioned and burn off the kitten energy. And I started teaching him the command for "come." I hope you got some ideas from what we are doing...I can update on the training and chicken interactions in a couple of weeks if you'd like.