mini humbucker in a p90

I think most of them are, if they follow the LP Deluxe mini-HB form factor. Not so sure about Firebird minis though.
You'll need a mounting ring, and assuming they're soapbar, you'll have to drill new holes into your body.

Infact P90 pickup routing is universally applicable to Mini humbuckers with plastic mounting rings, metal mounting rings (as for Firebirds) and even Gretsch Filtertron type of pickups.
Gibson made them in the first place to fit in the P-90 routes in all of their potentially scrapped Pauls as soon as the P-90's stopped being popular...