Re: What kind of magnets should I order to swap in my Phat Cat?
Yes they would fit. I always start off with... A5/A4 in the neck and A4/A8 for a beefier bridge.
The Blueman P-90 magnet set! I came up with those after being disappointed in my Phat Cat's stock A2's (to me, the wrong magnet for bridge and neck). Since P-90's have two magnets, you can dial in EQ's more precisely than with a humbucker, so I like to take advantage of that and use two different kinds in a P-90.
To the OP: I wouldn't bother with roughcast A2's, as the problem with the neck PC is it's overwound and pretty warm, and roughcast magnets take off a little high end. They desperately need brighter magnets to get some treble, mine sounded like a pillow was over the speaker. I tried an A5/A2 first and that helped, then an A5/A3 which was better still, and finally an A5/A4, which gave a nice sharp high end, a little more mids, and didn't have the bassiness that twin A5's can sometimes have.
Zhang's said bridge Phat Cats sound 'weak and puny', and I agree. They're overpowered by the neck PC. I thought an A8 in the bridge would be nice as it adds output, midrange, and warmth. An A4 keeps that going without pushing treble or bass. That magnet pair's worked for me for years. BTW, Zhang's solution for his neck PC was to take off about 1,000 turns of wire, which made it brighter.
