Question about baking 2 different clays

Hi, I've got a bunch of Sculpey III blocks and scrap, but I don't use it.
I'm using Premo and Kato Clay. Sculpey bakes for 15 minutes and the others
for 1/2 hour per 1/4"

My question is, can I use the Sculpey III for the innards if I'm covering
beads and other stuff with cane slices of Premo or Kato? Or does the time
difference for baking make it inadvisable to bake an object with a core of
Sculpey III for 30 minutes or more? I'm afraid it might either a) crack, or
b) release toxic fumes.

Thanks for your responses,

Martha