You'll probably find most guitars have their pickups wired in parallel to one another. There are a few exceptions which are series-wired as standard (I'm led to believe Brian May's Red Special is one famous example), and quite a few guitars which come with switching to allow you to go from parallel wiring to series. Fender's Baja Telecasters are notable for having an "S1" switch, which does exactly this!
As for pickup types, you're best sticking to single-coils for series wiring: two single-coils in series will tend to attenuate higher frequencies and emphasise mid-range - essentially, you've created a very spread-out humbucker, and the guitar will sound as such. Typically, humbuckers already have their two coils wired in series, so there's probably not much to be gained from sticking two of them in series with each other. Unless you're aiming for an unfeasible amount of lower-midrange woof, of course...