Those people above are correct. However, sometimes people do add crosses in, to increase the hybrid vigor of the flock. Crosses tend to be stronger to a point. Too much mixing and you really start loosing the positive strengths of a pure bred breed. Sometimes in order to get specific trait of a breed, there is too much in line breeding and other less desirable traits show up, that is where another less pure crossing can help.
So if you like your roosters, I would not hesitate to put them on pure bred hens, knowing that some of the eggs you hatch would not be for future breeding, but that is true of any breeding program. In two of three years, buy a good quality rooster, and I think in 5-6 years you would have a very nice flock.
Often times it is hard to give advice on this forum, because set ups are so different, from a mini farm to a true backyard. If you are planning to show, I would not recommend this. There is considerable difference in a private flock for the enjoyment of the owner, and people that sell and show purebred birds.
Personally to keep 2 roosters, I would want between 20-30 hens.
Mrs K