Re: Phat Cats in SG
I have an Epi SG Custom that I put a C5 on the bridge, a Phat Cat in the Middle, and a '59N at the neck. Pretty good combination. If I do another HHH guitar, I'll put a Phat Cat in both the neck & middle.
P-90's in a HB size are a great idea. The twin magnets let you pair up dozens of magnet combinations and really get the tone you want. Phat Cats use twin A2's, which are mid-heavy, and can be pretty dark in the neck position of mahogany bodies. I like to swap one A2 for an A5 to give it some sparkle & life.
In a couple of my 2 HB guitars, I've put in a neck Phat Cat (A5/A2) with a HB on the bridge, and because it's a single coil, it gives a clearer tone than a HB, and avoids the potential for muddiness.
If you're feelling creative, you can improve on the stock Gibson wiring, and with a couple of push-pulls, add coil cuts and phase. I have a wiring system I developed for my SG Custom, that I'll give you the details on if you're interested. Some guys here use a push-pull as a shut-off for the middle PU, to get it in & out of action faster.