The only thing I could see, and it has nothing to do with mixing brands, is how they voice together. There's Dimarzios that don't sound good with other Dimarzios, and SDs that don't sound good with other SDs. I had an SD Screamin' Demon a few years back. I didn't care for it by itself, but it sounded horrible with other SD pickups. As far as mixing brands, if it sounds good, it sounds good. The original Strats had 3 way switches that didn't mix any of the pickup sounds together. Seeing how none of them were voiced together, they could all be very different sounding pickups.