If I am stacking and I have a 12" bottom should I put a 10" or 8" on top of it? Which looks more "size appropriate"?
You can use either size, but the 8 inch would look better because you will have more room on the sides. I have stacked an 8 inch on top of a 10 inch and it looked fine.
It depends on the design but usually a 4" difference is best.
Either would look fine. 8" would give you more room to decorate the "ledge", but you could do either one. How many servings do you need?
How many tiers are you planning total? If you are only having two tiers, I think at least a 4" difference would look better (8" round). But if you plan on doing 3 tiers total, you could do a 6 - 9 - 12 or something like that.