ehh, I'm only going to say this one time.
DIFFERENT SIZE SUBS WON'T CAUSE CANCELLATION!!!
it will however cause an irregular frequency responce, which still isn't benefitial, however, it's not inherently too much worse than porting subs, it's just less predicatable. All that said, however, I can't say I can reccomend mixing sub sizes. Seperate chambers is always a good thing. It'll add bracing to the box. Plus if your 12's were to suddenly break while running alot of power to each sub, your 8's would suddenly be running free-air with WAYY to much power on them, now your down 4 drivers instead of 2.