You can mix different value potentiometers. A 500K volume + 300K tone would have a lower impedance than two 500K potentiometers, but a higher impedance than two 250K potentiometers. It's something worth experimenting with. The industry standard is 500K all-around for humbuckers (Gibson being the exception, using 300K), and 250K all-around for single coils. You can break those "rules" though. The JB/Jazz combo works very well with 250K pots - some people even prefer them this way. I also like to use 500K pots in telecasters - it really opens up the telecaster neck pickup, and gives the bridge even more bite, which can be tamed with the tone control to get a really snarling midrange-heavy tone. Then you get guys like Tony Iommi, who use 1 Megaohm volume potentiometers with disconnected tone controls. This is a high-enough impedance that it's practically the same as wiring the pickup straight to the jack. This works well with darker humbuckers, like the Iommi signature pickups. There's also the preferences of linear versus logarithmic tapered potentiometers.
The industry standard is logarithmic all-around, but Gibson is the exception using linear volume pots. I personally prefer linear volume pots, because it acts more like a gain control - letting you clean up the signal with just the volume knob, without really affecting the volume too much. Logarithmic volume pots will also clean up the signal, but make things quieter. It comes down to preference.
