I'm assuming your Bondo is basicly the same thing as our P38 or Easysand, A polyester resin with chalk dust ?
I've been using it in the motor trade for many years & on my foam based project. I also would have thought it would melt Styrofoam being styrene based, I used insulation foam on mine & PVA to seal the foam prior to using the filler.
I've personaly not found the need to add more resin but you can reduce the curing time by using slightly less hardener & as said build it up in thin layers. I also always take advantage of the Semi cured time when its setting but not quite set its a bit like cheese, when its like that you can carve it really nicely to knock down the sanding work.
I always use 80 grit aluminium oxide paper for the bulk of the shaping & then a finer 180-240 grit to remove the 80 grit scratches.
Also if your covering a larger curved area use a rubber spreader to apply it & Dont play with it too much looking to get the perfect finnish first go.
HTH