If you want them to be peaceful...breed your fish in a large tank, remove the female, leave dad with the fry and they should stay peaceful.
Each fish is different, I doubt you could just breed calm fish and make them not aggressive. They've been bred for aggression for the last 800 years, it'll be a while to get it out of them.
Who am I? Well I am a Junior Member of the International Betta Congress and a breeder of white HMPKs, red dragon HMPKs, black/red dragon HMPKs, metallic multi HMPKs, blue masked HMPKs, and now multi color HMs. Soon to add black HMs. I will be showing in New Breeder class in fall of 2012.