why does it have to be sugar free?
i dont think fondant (or marzipan) will work out or this cake. the majority of either recipe is icing sugar.
there are frostings you could use instead (ie: splenda based), or ganache (although i guess chocolate contains sugar, but then you arent adding any extra - would need investigating - and the same would apply to modeling chocolate, although i dont know if u really want to use that to cover a cake either.)
Kellie0406 besides the PM I sent you also remember any suger free chocolate you get @ a supplier can also do the same thing as sugar overload, to much sugar free of any sugar free product can also cause problems.
I tried Michele Fosters fondant and it was fine in very moderation for me.
Have you thought about Vegan websites there seems to be a lot of them doing cakes and desserts. It does not hurt to look to look at them maybe they can offer you some advice too. Good luck.
I'm a diabetic, and I've been looking for a sugar-free fondant recipe, but they just don't seem to exist, which is why I was thinking, what if I used Stevia instead of sugar? It's not sugar, but it sweetens just like it, why shouldn't it make a good fondant? And if that doesn't work, I can always just peel the fondant off before I eat the cake. Simple!
I have a few tester recipes that I edited from regular fondant. They all required powdered sugar and shortening, so instead of shortening (if your recipe calls for it!!) use butter, and then for the sugar you can mix 1 cup Splenda/Stevia with 1 teaspoon corn starch in a food processor until the texture is like that of powdered sugar, and you will have to scrape the sides down a several times, so ALL the Splenda/Stevia and corn starch is processed. Use this in place of powdered sugar, and granulated Splenda/Stevia in place of regular sugar. I haven't had a chance to try it out in any fondant recipes though. I hope it works! Good luck!