I need to make a fondant dummy cake for rental for a wedding shop. I know I will need to harden the fondant as much as possible, I usually add gum tex when I am doing my figure modeling and have never tried adding gum paste. Is gum paste my best bet? If so what would the proportions be?

I have limited experience with fondant or gum paste, but I would use 50/50 fondant and gum paste or try adding some tylose to the fondant. I haven't had much luck with gum tex making the gum paste recipe or adding it to fondant. But that's just my opinion and I am far from an expert!

you can use straight fondant... but it will take about a week to dry completely...

I just use fondant by itself. I find that it sticks well to the dummy if I roll it out in shortening to keep the surface of the fondant moist. Careful as the styrofoam dummies can be a moving target when covering with fondant. Be sure to put a non-slip surface underneath. Have fun!!

Straight fondant is fine. It takes 4-5 days to dry completely. You can attach it with piping gel or just wet the dummy with water.