Phosphor bronze strings are non magnetic, but the E & B are. So it will be hard to get a good balance using acoustic strings IMHO. They will work but I think nickle would sound a lot better when you plug the guitar in. Its a tough decision but I think it comes down to a few considerations.
#1 How often will you be playing this acoustically? If the amount of time is significant then the choice should be for a good acoustic tone & IMHO Phosphor bronze work better for this, but if you decide that the guitar will be used mostly plugged in then go with nickle. They will sound louder and you will get a better, more even balance.
#2. Why did you go with a P90 in an acoustic instrument? There are so many wonderful acoustic solutions that would have worked much better AND you would have been able to maintain a really good acoustic sound amplified. LR Baggs makes some wonderful acoustic pickups as well as Fishman and others.
I have three acoustic guitars that I can plug in. Two have the Fishman Eclipse system which has a Piezo crystal under the saddle & a volume a tone control mounted inside the soundhole. I can also add an internal microphone if I need to but so far I have not had any situations where I would need it. I have these in a Martin D15 & an Alvarez YB1 Baritone acoustic. I also have a Fishman Piezo pickup in my Martin (Sigma) D18. All three sound really nice through a good PA system or my Shenandoah acoustic amp. I just use my standard D'Adarrio J17's on these guitars & the Barritone also has Phosphor bronze on them(not sure of the model #).