Does anybody have a picture of a cake with this shape configuration?
HEre's a square/round/square: http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1341709
When stacking a square on top of a round, be sure you measure first. For example, a 10" square is 10" from side to side, but it's about 14" from corner to corner. So if you tried to put it on top of a 12" round, it would have some overhang.
Another helpful tidbit...when mixing shapes, I build it with pans, then you can see just how each will fit on the next.