iamadoorknob

My main strat is a fat strat (HSS). The middle pickup is reverse wound, but when I use the position with the humbucker and the middle pickup together, I get that 60-cycle hum. This happens whether or not I'm coil tapping the humbucker. The humbucker is a Seymour Duncan SH-5 and the middle pup is a GFS Premium Alnico Vintage Strat pickup.

I'm not really concerned about that right now, but what I'm concerned about is the new guitar I bought. It has two P-90's in it, and I'd like to swap the bridge P-90 out for a Seymour Duncan SH-12 humbucker. Right now the guitar has a pretty nice sound from the middle position and it's humbucking, I just want to sure that if I buy a new pickup that I'll still get humbucking out of the middle position (humbucker and P-90). What is the problem in my main strat, and how can I avoid it on my new guitar?