it is a combo of both nature and nurture.... more hens and more space will lesson aggression, but some breeds are more prone than others....
i recently took in 2 sumatra roos from an urban byc keeper. besides the crowing, they were fighting. i was clear, if they continued to fight, i'd have to rehome or slaughter the more agressive one. i have many hens, and an established roo. Happily, the newcomers stayed quiet for some weeks, then staked out their territory and hens... they patrol their territorities, with some overlap from the established roo, but no fighting. crowing is moderate.
i've done similiar with mice and other rodents.... fanciers 'accept' the males will be territorisl.... urine mark and fight, sometimes to death.... if they did, i retired them and their offspring (eg snake food), or used them very carefully if they had other rare, valuable traits..... if they did not fight as much (and this is a sliding scale when you start out), i used their offspring.... i soon had males living in 'bachelour cages', i could remove one to breed, return it , and no fighting.... what is sometimes generally acceptable by tradition doesnt hold up to application.... if one is willing to try.