If you want a mature cat, look for a small mature cat. I had a full grown cat that weighed maybe 2-3 pounds. She was an excellent mouser, and was too small to do any damage to bantam chickens - and never bothered the chicks. However, I'd still watch a full grown cat around the chicks at first.If a kitten does better we will probably have to look for a kitten/younger cat.
That's exactly how my older cats are.Just my own experiences -
I had a kitten - about 7 months old when the chicks arrived - who seemed to LOVE them. He slept under the brooder when they were small and never showed any sort of aggression toward them. When they got bigger and moved out to the coop, he slept against the fence and the chickens would gently pull at his fur through the wire - like they were preening him.
The other barn cat - probably 8 or 9yrs old when the chicks arrived - is a fierce hunter. But has never showed any interest in them - doesn't seem to even notice that they're there - even when they were chicks.
I probably just got lucky, but my cats never seemed to make the connection - chicken=bird.