Is there a general rule of thumb when trying to do this, or should you stay brand specific? For instance if 1 pup has D.C. =13K and another has D.C. = 12.39K, regardless of brand can they be paired up?
You can use pu's with different resistance no problem, especially when the difference is so small as in your case. Even if it's a big difference, you can do it, the only problem being that when both pu's are on at the same time, the one with lower resistance will contribute more to the sound than the other one, because of the mutual loading between them (to put it simply the lower resistance pu "short-circuits" the other one more than the other way round), but you can always compensate with the vol or blend controls. Finally, this is all regardless of brand, you can also have pu's with different resistances within the same brand, so it's not a factor.