Most flocks contain roosters. If you want to sell your eggs, it is a bad idea to keep roosters with your flock as they will fertilize the hens. Some people have incredibly sensitive stomachs and cannot eat fertilized eggs, no matter how developed it is. If you still want roosters, be them for show, a more natural look to the flock, etc., then you will need to keep them separate from the hens.
So how do you do that without building a whole new run for them?
Build a small coop that can hold the amount of roosters you want to keep. Buy three or four PVC pipes and bend them into a 'U' shape. Hammer the ends into the ground, making sure hey stick. Then attach basic chicken wire, making a small, convenient run for the Roos. Make sure they have access to greens! You also will want to put chicken wire around the base of the coop to keep predators out. The roosters will alert the flock when danger is nearby and crow, but will not be able to mate with the hens. Also, they shouldn't fight together; if they do, move the coop and rebuild the run out of sight of the hens. They will be able to hear them but won't be able to see them.
This also helps with controlled breeding; shut the roosters into the run while you put a female in with the male. Make sure you can see what they are doing so you know whether or not the hen is fertilized!
I hope this helped!