I was planning on using Indydeb's buttercream under the fondant (instead of using marzipan sheet) Is this OK?

Sure, it's fine.

Generally, marzipan is only used under fondant on fruit cakes, not on sponge or American style cakes.

Rae

thank you for the reply. This is my first try at fondant and it really helps to get some sage advice.

I only use fondant on a frosting layer that I can get very hard and smooth: ganache, SMBC, or an all butter ABC. I believe that that recipe is a crisco based, which while being heat tolerant, will never get as hard as a butter or ganache. I find using a softer ABC under fondant makes my corners look mushy and more prone to sagging and movement under the fondant when smoothing. Maybe it just takes a lighter touch than I have to make it work, but I much prefer being able to vigorously smooth and work the fondant onto a cake.