hi Mike
we also have a yorkie cross and she is great with the girls, mine freerange all day and the dog comes and goes through the cat flap all day, it probably helps that we also have four cats so she is used to other animals, she never chases the cats or the chooks, every dog is different, we did keep them in their run for a couple of days then let them out and kept her on a lead just to be sure, we have had the chooks 5 months now and she has never chased them, hope this helps.
littlefeat