Would it look silly to stack an 8inch round on a 9inch round? This is for a stars & stripes cake - red stripes on bottom, blue top with white stars.
I tried searching for photos of the combination of an 8 and 9 inch, but couldn't find any.
I don't think so, ther are so many variations of cake that usually anything goes (exception wedding cakes) lol... but if your comfortable with it and you know what you want it to look like just do it.
An 8 and 9 inch round will not give you much space for any decorative elements. I would consider your design and how much space you may need. When I make two tier cakes I usually have my tiers three to four inches in difference to allow for enough space. I also feel these size differences make the two tier look more complete and not like another tier should be on the top.
After stacking an 8" on top of a 9" there is only half an inch on either side, so it might end up making the cake look like one tall cake and if you plan on making a border (depending on what kind of border) if might not fit.