When people ask about how to get the rooster just to breed certain hens, there is often two thinking behind that question, either how to get pure bred eggs, or how NOT to eat fertilized eggs.
If you have a mixed flock of different breeds, introduce the rooster of the breed you want, and only hatch the eggs out of the hens that match him. Usually you can tell which eggs come from which hen with a little practice and observation. If some are of close color and shape, take pictures and mark them. You can either put your chosen eggs under a broody hen or in an incubator.
If you are worried about eating fertilized eggs, don't be. Fertilized eggs are rather difficult to spot, and take some education and experience. Most people will not know if they are fertilized or not. Another point is, chicken eggs are not like mammal eggs in which the egg begins to develop upon fertilization. Chicken eggs are in limbo, a state of suspension, kind of like a seed, and will not begin to form into a life form until certain conditions are met. This allows a chicken to lay an egg a day for several days. The eggs just sit there doing nothing until the size of the clutch is right, and she stops laying eggs, and settles down on top of all the eggs and begins to brood them. If the eggs are kept at nearly 100 degrees for more than 24 hours, the embryos begin to develop. This allows the last egg laid, and the first egg laid to develop at the same rate, and hatch within minutes of each other, not days apart, where she would either leave the not finished eggs or let the live chicks leave the nest. They all hatch at the same time, and she starts taking care of the live chicks.
If you get your druthers, get a rooster as close to a year old as you can. Don't worry about it, and just see what happens.
Mrs K