Is a 6, 8, and 12 ok for a stacked wedding cake? They only need to serve 70 people, I just wondered of the bottom tier sticking out more than the other tiers would be considered off balance. A 10 inch bottom wouldn't make enough servings. I have round pans.
I don't think the difference between layers needs to be identical...as a matter of fact...I find it more interesting when it is not!
Wilton's Wedding Book say that each tier should have a difference of 4" because of the balance in look when your eyes go from bottom to top.
If it looks good to you, then use the larger bottom...as if someone is going to stand there and say hey, there is not 4" difference for each tier. lol
To me, I think the larger bottom would look great.