JUST A THOUGHT:
Whatever breed you buy, buy big. My dog, Xena (since passed
) was afraid of other dogs as a puppy, but NOT afraid when she grew to 105 pounds. Yet, she was very gentle with the cats, rabbits, and chickens.
If you haven't been a dog owner, do some research, AND buy a
puppy!!!
I gave up trying the retrain one dog who could not be contained---the owners thought he needed to live in the country--HA!!!
and another who had been abandoned after taught to attack.
A short search on the Net will tell you about certain breeds and their genetic predispositions. Some are great for your purpose, others could also work. There are even some who will buddy up to cats, for instance (who CAN defend themselves, unlike chickens) if they grow up with them, but will chase and attack them if not exposed. So, you could train one of these. I believe that every animal you own--I used to take care of 21 on my property!!--needs to have a job in order to be sane.
IN ADDITION, there is no guarantee in the dog kingdom. My vet's most aggressive client was a collie. But, I came very close to buying a property that came with the dog, a Doberman, who didn't want to leave with the family---VERY steady and friendly.
I'm sure that you'll figure it out!

Good luck with your new chickies---mine have already arrived---oops! gotta go check on 'em!