First, I was wondering if it's alright to put passive pickups in a guitar that currently has actives. That being said, I was thinking about puting a Seymour Duncan SH-11 custom in my Schecter C-1 at the bridge, and maybe a P90 in the neck position. What are your thoughts on this?
as long as you replace both actives you should be fine
you're guna wana get a humbucker sized p-90, becuse a reg p-90 wont fit in the neck pickup spot of C-1, since it is routed for a humbucker