I know this is probably a dumb question and I haven't done much with fondant....so, can I cover a cake with fondant and freeze it???

Also, can leftover fondant be frozen?

FondX fondant can be frozen.

On the cake, just leftover, or both?

On the cake. It's a commercial fondant so it won't really go "bad". I think most commercial fondants have a shelf life of a year?

What about marshmallow fondant? Can I put it on the cake and freeze it for say a couple weeks or a month?