No, a humbucker route cannot accommodate a single-coil pickup or a P90, if you're talking about a strat or tele pickguard. I've tried it and had to do extra routing. Warmoth offers a 'universal rout' which is big enough to accommodate any type of pup - this is not the swimming pool route, but a big rout for one pickup.
A humbucker route CAN accomodate a Fender single coil sized PU no problem; P90s are meant to be mounted to wood and while theoretically you could mount them to wood and have a pickguard custom cut for that hole, lining them up could be a pain; you're better off getting a P90 in a humbucker mount, there are a number of different mfgr.'s P90s available in that form factor.