Can I use a 6" and 10" plates/pillars on an 8" and 12" cake?
Yes you can but why would you want to?
Ha! I'm doing a 5 tier for Tuesday, I don't have time to order more and the tiers those missing plates were supposed to be on are crumbly chocolate and I'd rather not use dowels for those. The other sizes are sturdy vanilla and I could dowel those without worry. I'll be ordering more, but they won't help for Tuesday's cake.
Can you get foil cake boards or drums the size of your cakes? If the cake layers are on the larger boards just add a little frosting to your sps plates to act as a glue.