All of our young roosters (cockerels) for sale have been handled and show no signs of aggression.  Adding a rooster to the flock has several advantages:
Roosters will alert and protect hens, especially against aerial predators.
Roosters will police and break up fights between hens. And of course, roosters are essential for fertile eggs if you want to brood and hatch your own chicks.
 
We enjoy watching our good roosters interact with our hens. Note that you should have at least 5-7 hens per rooster to keep everyone happy.