Hi and welcome to Cake Central!
I don't see why you couldn't use a canned frosting under fondant. I think it would be fine.
As far as the MMF (marshmallow fondant), I have not tried it yet personally, but a lot of people on here like it. I have tasted it, and the taste is way better than purchased fondant in my opinion. Some people say it is hard to work with though. I'm sure someone else can give you more information on this. Good luck!
marshmallow fondant is great!!! It's cheap to make and I don't have a hard time with it. Sometimes I'll add a little more water to the marshmallows depending on the weather. Also depends on the size of the bag of marshmallows too.
http://www.cakecentral.com/cake_recipe-1949-Marshmallow-Fondant-MMF.html
here's the link to the recipe.(and directions on how to use it to boot)
I just started using MMF too, and OMG, it's AWESOME! SO much better tasting than the store bought variety. The only thing I don't like about it is the stickiness at the initial mixing stage. very gooey before you get all the sugar mixed in. BUt once it's mixed, it's great.
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