I try to keep to the one male per tank rule with my adult fighters. Its rare that they go for the outright kill but the strongest male will rip the other males body and fins more often than not leading to ones death. You can keep younger males together - I have had up to 100 4 month old males in a tank with no problems but they were from the same spawn and had never been separated.
I normally keep one male in each of my females tanks, with up to 30 females. He's usually fine, sometimes they will nibble at his tail. Females also can fight with each other, but nothing serious and I find they do this more often if theres only a couple in with a male or if theres a storm front passing overhead.
If I have only one male and one female together I find they spawn every 3-4 days which is not good for them.
Its really up to you and the individual fishs nature. The more plants and cover you have the more peaceful your aquarium is going to be.