I have used this method but I have to allow sections to dry before i can build on top of it so that it doesnt loss the shape i am going for. I have the best results using a 50/50 fondant gumpaste mix and add tylose to that. I can make a figure in one go and i dont have to allow for drying time.
I totally agree with Bella, it all depends on what you want to make. Fondant & tylose mix will give you a modeling paste thats just okay but sags under pressure. While fondant + gumpaste + tylose will give you a much firmer paste that holds its shape well (doesnt sag) and is more resistant to humidity.