That 10 to 1 ratio is for fertility only. That is what commercial operations that provide hatching eggs use in a pen breeding set-up to assure fertility. That ratio has absolutely nothing to do with roosters fighting and practically nothing to do with any bare backed hens or other overbreeding problems. Breeders often keep 1 rooster penned with 1 or 2 hens and usually don't have those overbreeding problems. Some people have problems with free ranging flocks with over 20 hens and only 1 rooster. Don't get too hung up on a ratio. Whether you have problems or not depends a lot more on the personality of your chickens than the ratio. This depends not only on the personality of the rooster but the hens too. I am a firm believer that the more room you give them the less likely you are to have problems, but even that is not a guarantee.
The more roosters you have the more likely you are to have problems, so I recommend you keep as few roosters as you can and still meet your goals, but there is no magic ratio of hens to roosters that either guarantees you will have problems or that guarantees you will not have problems.
From experience, I have had more problems with one rooster with 10 hens that I have with 3 roosters with 15 hens, and those 3 were highly sexed adolescents, which is where you are more likely to have problems. As long as you don't get ridiculous, from experience I've found that the ratio is not all that important.
Will you have problems with 2 roosters and 12 hens? Maybe, but maybe not. I really don't know.
