Sounds like you have it covered. All of my barn cats have come from local shelters. Each time they set me up with cats or kittens that they felt would prefer being barn cats rather than living inside. And eventually, all of my barn cats end up becoming house cats and then we get more barn cats. Barn cats definitely keep rodents down, as long as you feed them. They can't survive on the rodents they catch. In fact, they will do better if they kill the rodents but don't eat them. None of my barn cats ate rodents, to my knowledge, but they present me with their bodies as presents, and I praise them and give them treats.