A good rooster will protect the girls, giving his life if necessary, ensure they have food, make sure they eat first, alert them to dangers, and fertilize your eggs. That said, there's no requirement for a roo if you choose not to have one. You just won't be able to hatch your own eggs. Integrating a new roo is no different then adding birds to the flock. Start them in a shelter inside the area where the girls are for a week or two so they can get to know each other through the fence. Then release him but supervise. Depending on the age of the hens and the age of the roo, it might be wise to watch some chicken porn on youtube so you know what to expect.