"The Greatest Evil"

I feel like we skimmed over a very important contradiction back in chapter five. This contradiction of plato is related to the "Guardians."

On page 130, line 451 e Plato via Socrates says, in respect to the guardians, they shall have "Everything in common." On page 141, line 462 b Plato (or socrates) says "Have we any greater evil for a city than what splits it and makes it many instead of one?" "No, we don't" was the response to the question. With these words it seems clear that creating factions among the guardians would be at the very least "evil." The contradiction arrives when Plato and his characters decide how the guardians should reproduce with each other. They do exactly what they have agreed is crucial not to do; split them into groups. "There is a need for the best men to have intercourse as often as possible with the best women, and the reverse for the most ordinary men..." Plato is creating factions among the guardians...? How is this possible. As we mentioned in class, Plato's excuse for all of this is his "Noble lie." "Our rulers will have to use a throng of lies and deceptions for the benefit of the ruled." Certainly the "Greatest evil" for a city would outweigh a noble lie which benefits a certain faction which makes up the smallest percentage of the city. None of this makes any sense. Am I missing something here?