Re: mini humbucker replacement
Yep.. Gibson mini-hums and P-90s are exactly the same size. I just replaced the mini hums on my LP Deluxe with some duncan P-90s. No problem at all.
You can mount them by screwing directly into the wood (as I did) or use the mini-hum mounting plates.
I think some early 70's SGs had different, "squared" mini hums that may not be as easilly replacable - also not 100% sure about all Firebirds, but i know it works for the Les Pauls (Special, Deluxe).