I see nothing wrong with a flock of all one breed, and I have seen it done. If you only care for egg production, that is what people usually do. I personally would be bored having just on breed. Part of the fun of having chickens is acquiring all those unique, different breeds of chicken from bantams to colorful layers.
In our flock we have 2 Buff Orps, a Wyandotte, 2 Sex links, 3 Easter Eggers, an Australorp, and a Speckled Sussex as the main egg producers, and they are all shades of the rainbow. Then there is a pair of Belgian D'Uccles
( beautiful bantams btw) , a white Silkie rooster, and a black and white Serama rooster. For added flavor, a Coral blue guinea hen ( male) soon to be joined by yet more guineas. Of course I have ducks too, Calls. They are icing on the cake. The ultimate collection would include at least 3 peafowl, but I am holding off on that for now. Sorry for rambling, but I love talking about my chickens!!