To improve upon this above answer:
Actually, this isn't completely right. I've been playing classical guitar professionally for 30 years and many classical guitarists, including myself, mix "brands" of strings. I might love a particular brand of bass strings (strings 4, 5, and 6) but hate that brand's trebles. I know of players who even prefer one company's "b" string over every other brand, and that's the only brand of b string they will use. Classical guitar strings are all pretty much the same gauge, but tensions and materials vary, (composites are making inroads) so that might be a characteristic that you might want to keep consistent, no matter how many different brands of strings make up a full set for your tastes.
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