MzCuteCupcakes – a 10" and 8" cake cut into 1 x2 inch slices will feed 82 people. I use this site to help me figure that kind of stuff out (http://shinymetalobjects.net/cake/calculator/cakulator.html)
as for the different shapes together I like to use dummies to stack and play how I like the different combinations. I try to keep one of each circle and square in sizes 6-10 to help with that. For me I am not sure if I like the top being a circle. If I was going to use mixed shapes I would put the circle in the middle, but that is just me.
And for your 3rd question maybe you could cut it out of fondant and then place on the side of the cake
