The Platinum P90s will be the same dimensions as standard humbuckers, so you can either use the kit humbuckers routs as a guide or use one of the many templates available off the web.

Copper pickup rings and control plate won't be an issue as long as it's thick enough. I'd suggest 2mm minimum as it's quite soft. But I'd definitely solder the rear of the pots together with wire for grounding, rather than rely on the copper. Unlike a chromed control plate, copper will oxidise over time, and the oxide isn't conductive. So although it may start out clean and shiny on the underside (or maybe not if you are going to age it first), you'll probably find that the controls become intermittent with time without the hardwired ground connections.