I would like to do a Elmo cake with the 1/4 sheet cake covered in fondant and the elmo shaped cake on top decorated with butter cream icing. Will this be okay? Will the buttercream affect the fondant in any way?
You should be ok, but I would probably put the elmo on a piece of cardboard the same size as he is. You wont see it and then you wont have to worry about the fondant at all.