Already been said, I will say it a bit more clearly. It's about genes...Nothing more, nothing less. You didn't really mention what kind of aggression you are hoping to avoid though? First few months of their life they will be or typically are obnoxious like a teenager and slowly mellow out into a rooster. Not sure the age of yours, but don't judge a cockerel on his pubescent months. I've seen 'pet' roosters turn on their owners in an instant after several years, so you never really know.
-When you do see him/them make sure they get out of your way. This doesn't mean get all hillbilly on them. Simply just don't walk around them.
-See what his dad is/was like. You may be able to get a potentially better idea.
-Respect what he is and what he was created to do. Fowl are complicated creatures, more so than most people realize. Sometimes rooster aggression towards people is guess what? Something WE are doing wrong that he finds threatening.
-Don't apply human emotions/conditions to a rooster.
-People keep animals, sometimes for a lifetime. Doesn't mean they really know anything about them. Keep that in mind. Do YOUR own research, and believe me there are tons of articles about the dynamics of roosterdom.
Roosters are amazing, a great rooster is spiritual. Good luck with your boy(s) and I hope they grow up into amazing birds.
There are a few rules that I expect all animals-- regardless of species!-- to observe around me, and that is no fighting amongst themselves when I am in the 'room'. That means when I approach my horses/cows/chickens with food/water/treats/attention, I will
not tolerate the big/dominant ones beating the smaller/weaker ones away from whatever it is I offer, nor to I appreciate a 'bubble invasion ' where everyone competes to be closest to me. That is not rooster specific (obviously lol) but it is behavior I discourage.
Any form of human aggression will not be tolerated. I live on a very safe dirt road, and the neighbors very young children like to ride their bikes, walk their dogs go exploring etc past my house, and if either rooster attacks a child, it would be bad. Also I have young neices that enjoy the birds. I don't allow anyone to chase or pick up the birds, so there is no reason for a human to be percieved as a threat to their flock.
Attacking the dog is not a hard no, but I would like to discourage it. Freddie, bless her heart, is a sweet and loving dog, but a hound, and dumb as a box of bricks, and if it moves too fast none of the training in the world will stop her from chasing it.
I won't allow any sucker punches with the dog on the receiving end, but if she chases a hen and gets attacked, well. . . Maybe she won't do it again!
So far, she is heavily monitored around the chickens, and NO chasing is allowed, but I won't say she won't ever chase them. She's a dog, and not a particularly smart or obedient breed either.
Overall, I think Nardole (polish crested) has the potential to be an EXCELLENT flock rooster. He has started dancing for the pullets, and tidbitting for them, but when his advances get rejected he does not get forceful about it. He constantly monitors the area for potential threats and alerts his girls.
I am quite fond of the ridiculous little guy, and hope he sticks around!