You can, but you have to know what you're doing else you'll be locking one end before the other. As a rule, you want to have most of the stopping power on the front wheels, but you don't want to go too extreme.
I'm running PF discs & pads on the front and OEM discs with PF pads on the rear with braided hoses and Endless fluid, and it's a very good setup for fast road and moderate track use. Great bite from cold and zero fade on track even after 20 mins, the only obstacle is the massive weight of the Zed which means that whilst stopping power is better than OEM (along with feel and fade), you're still stopping 1700kg+ of car (incl driver + fluids) which means that it's not quite as YOINKS as I would like. Obviously the next step for me would be for a larger setup with more pistons up front and maybe slightly larger discs in the rear, but then I'd need new wheels possibly and you start getting into daft costs considering what I've spent already. For maybe 90% of people I'd say the setup I've got is on the money, but if you really want to push harder then a BBK and/or serious weight loss (200-300 kg MIN!) of the car is the way forward.
All IMHO, YMMV etc.