Cockerels and roosters are a crap shoot. A lot of roosters start off as darlings, and people just love them. Then they turn into different birds. Often times these birds are not darlings, but rather nightmares. Inexperienced people cannot really believe this will happen. I think it is because if you have friendly puppies and kittens they turn out into life long companions. This is NOT TRUE for cockerels and roosters.
There is also the misconception, that if they are raised together, they will be life long friends. That also is not the case. Some roosters get along. Some roosters get along for a while, and some roosters will fight to the death. They don't call it cock fighting for nothing.
So do know, that even if you build it "right'. Even if they have been best friends and got along well. There is a good chance that the bachelor pen will not work for all of them. Do know that todays behavior is no indicator on how they will act tomorrow. They are very unpredictable.
IMO - Roosters need more room than hens. Build a coop/run that has a lot of clutter in it, out of sight of the hens. Some people have very good luck with these, some this works until a butcher weight is achieved. However, with roosters - you will need a plan B - a dog crate, a fish net in order to catch and separate fighting birds if needed. Most inexperienced people vastly underestimate how violent and aggressive roosters can be. Some are not, but a lot are.
Good luck,
Mrs K