I can't honestly say that I have ever noticed that chickens are racist, or breedist, or whatever you'd call it. I really don't think they care much how many of what kind of chicken they hang with. That said, I don't really like introducing single birds into a group; I prefer that a newby has at least one friend. I feel it reduces the stress on the bird to have at least one other bird that it knows it can hang out with, and tends to spread any unpleasantness around enough that nobody gets bullied mercilessly. If you were getting chicks, I'd say get whatever you like; they'll all grow up together, but if you are talking POL pullets, I'd prefer pairs, too.