I am making a 8' cake on 10in plate. 5in. spiked pillars. 12 in. cake on 16in. cake plate. On top of the 8 in. cake I am putting a gumpaste flower arrangement with a 5 in. styrofoam ball cut in half as my base for the flowers. What should I do for the bottom cake. More flowers of just put some color in the border to incorporate the colors of the flower? Hope that makes sense. This will be a buttercream cake.

If you are talking about the 12 inch cake, I would leave it plain. But that all depends on the style you are going for. Maybe you can add some color to unify it.