I am new to working with fondant and need to know if I can cover a cake with non crusting buttercream under the fondant or if it needs to be crusting?

Hi, you can use non crusting buttercream under fondant. It doesn't have to be crusting.

If you use a non crusting buttercream, I would put it in the freezer to stiffen up while you roll out your fondant. Other wise I find it is too soft and it oozes out the bottom sometimes. Other than that, no problems. Use whatever buttercream you like.

I use a crusting b/c and have found if I freeze it about 30 min before I put my fondant on, the b/c will not ooze at the bottom.