Hey Phil, welcome to the BKP-forum!
A P90 coupled with singlecoils has been done in many guitars and can be a great combo.
You should only beware of the balance between the singlecoils and the P90. Something like a Stockholm and Mother's Milk would provide a disbalance, as hotter P90's can sound very powerful and big, even bigger than humbuckers with the same output.
Then about the Pacifica. I had a few myself. Great, quite thin necks, good playability, but often a slightly thin tone.
I think you're PAC will benefit from pickups with an enhanced midrange.
Here are a few combo's that imo opinion will work fine and are more or less in balance.
P90/single coils:
Mississippi Queen - Irish Tours (slightly hotter, more bass, less treble than Mother's Milk, but they clean up great, you can get close to Mother's Milk/RHCP)
Stockholm - Slowhand (overall beefier set, the Slowhands have quite some mids, but still vintage vibe, that will match better the huge bass and mids of the Stockholm)
Humbuckers/singlecoils:
VHII (noncompressed, good balance with a present, but not shrill topend) - Irish Tours.
Abraxas (beefed up Mule, more midrange), some compression) - Irish Tours or Mother's Milk.
Crawler - Irish Tours - The Crawler splits wonderfull and will turn your PAC into a fat, growling monster, with still good PAF-flavour. Split the Crawler for a tele-esque tone. Combines wonderful with the middle-IT.
Nailbomb (alnico version) - agressive, throaty midrange, huge bass - paired with Irish Tours.
Holy Diver - middy, creamy old school hardrock pickup, splits very nice - Irish Tours.
I would wire it according to the Suhr-diagram with a 500/550k pot for the humbucker and a 250k pot for the singlecoils.