I'm only answering this because no one else has yet. I'm not an expert by any means. Though I've used fondant quite a bit, I've only made one dummy cake with fondant. That was in a class. We didn't add gum tex to the fondant, just used it as it came from the tub. It dried quite well. I kept the dummy for 2 1/2 months and then threw it out because I didn't like it, not because it hadn't held up. In the class, when we wanted more strength in the decorations we added Tylose powder to the fondant. I think Tylose is pretty much just the the non-Wilton version of gum tex It makes the fondant act more like gum paste. It doesn't taste very good, but for a dummy, that wouldn't matter, so you could cover the dummy with it as well, I suppose. As you no doubt already know, it is possible to roll gumpaste out thinner than fondant, it is also more stretchy, but it is more brittle too. Gumpaste doesn't take color quite the same as fondant, so when you want to match colors, adding Tylose to already colored fondant is a way to get around this. To me the fondant with Tylose added is not like either fondant or gumpaste, but somewhere in the middle, but that may be becasue I don't really add enough Tylose.
Anyway, this is my round-about way of saying I'd just use the fondant straight, but hopefully someone who knows more than I do will answer you.