Re: P-rails hot
The P-Rails models do offer 2 humbucking modes, parallel & serial, and a fair number of people seem to like it's parallel mode more than other humbuckers. Other thing is usually the Sustain mode kicks in humbucking automatically, so you can still have variety the rest of the time.
Sustainiac can use the sustainer as an active pickup when it's not in sustain mode, too, which also improves versatility.
Not certain what I'd choose for a bridge pickup, depends a lot on the guitar and the range of uses I wanted from it.
Might look at a PATB-2 Parallel Axis Distortion or P-Rails Hot if it's for metal mayhem. PATB-1b for a broader range. PATB-3 if you want a bit more reminiscent of a PAF, but still hot enough to drive the sustainer adequately.
If actives appeal, maybe the Gus G Fire bridge [though I think it's only sold as a set, so that'd leave you with an unused neck model].
Or there's any number of more common models. JB, Distortion, Custom 8, etc...
I'd stick to something that's at least moderately hot, lower output models don't tend to work as well with sustainers. Outside that, really need a bit more on the guitar & what you want to do with it to give specific advice.