You can avoid this trap by measuring the DC resistance of a pickup with a multimeter. If one pickup is over double the resistance of the other, you can be fairly sure they won't mix well. Less than double will at least provide a noticeable difference, but of course identical pickups are guaranteed to combine best. If your guitar provides a separate volume control for each pickup, you can compensate for mismatched pickups, but I find this fiddly, and not as clear as a properly matched pickup combination. For this reason I prefer to have the pickups and switching come before a master volume control, and use the master volume control for level changes.
Polarity refers to the phase of the signal in relation to the string vibration.
At the other end of the scale you will find some pickups with their connections encapsulated in epoxy resin, providing only a single wire and shield. If you get a hollow, middley sound with a volume drop, you probably have pickups with opposite polarity, and need to reverse the connections on one pickup to restore normality. Combining pickups in parallel, but out of phase is less sensitive to impedance differences, and this may the best way of getting some useful variety if you're stuck with a guitar with mismatched pickups. Combining pickups in series is a matter of personal taste and experimentation, but I recommend NOT using similar sounding pickups. In phase it's just louder and mushier, while out of phase you will find a dramatic loss of volume.