I do agree that training trumps breed in maybe 80% of dogs. However, you stack the deck in your chickens' favor if you choose a breed (or predominant breed in a mixed breed dog) which has herding/retriever rather than hunting instincts. Look on the AKC dog website for breed descriptions.
Hunters: all terriers, schnauzers, dachshunds, beagles, schitzus (sorry for the spelling), chihuahuas, yorkies, spaniels, etc. These breeds were developed to nab and kill prey.
Herding/retriever dogs: collies, shepherds, rottweilers, shelties, labradors, retrievers, etc. However, many of these breed have had the quiet work instincts overbred out of them and they're hardly suitable for family dogs let alone being safe with chickens. And there are many stories of individuals of these breeds who have mauled and marauded through a coop! But at least you have a chance of training to an animal's strengths.
My family has owned rottweilers and dachshunds for more than 50 years, and my daughter in law next door has an Australian blue-heeler. The dachshunds have all delighted in killing any critter they can find. No way would I trust my chickens with a dog that strains at the leash to chase and kill. The rottweilers (once they were past 3 years old) were completely laid back and ignored chickens. The blue-heeler herds them into corners and then doesn't know what to do, so he lays down and just watches the girls and happily keeps them penned in a corner.
We've tried to train the dachshunds in many ways to stay away from chickens, but nothing has overcome the overpowering urge to grab it, shake it to death and gnaw on the remains. But we don't have any gophers or ground moles either, and the squirrels know better than to stick around when the dachshunds come to visit.
Try to choose a very young shelter dog so you can at least hope to be a major part of its training. Older dogs can learn new tricks, but it's best to start young.