I recently found out how to, and not to mix clays.
Q. Why mix clays together ?
A. For the same reason an artist mixes paints, to get a one-of-a-kind color. Sculptors who use polymer clay mix different clay colors to get a very unique color for their sculpture's skin tone.
I notice I'm doing it more and more now and I really like, if not, Love the results. You can come up with some very beautiful, unique shades of clay by mixing the correct colors together thereby adding to the one-of-a-kind aspect of your sculpture. Now onto the reason I'm posting.
I recently was working on a mermaid sculpture and I made her tail out of FIMO clay and her torso was Super Sculpey clay. Once she was ready to go into the oven, I cured her at the manufacturer's specifications, trouble is, I'm using two different manufacturer's clays on one piece. So I chose the safest and just to help you all out, this is what I did in case you all still want to mix your clay manufacturers;
Recommended Baking Instructions
FIMO: Bake at 230◦ F (110◦ C) for 30mins.
Super Sculpey: Bake at 275◦ F (130◦ C) for 15mins per 1/4 inch of thickness.
What I did was follow Sculpey's instructions and when it was done curing and cooling down completely I noticed the Green FIMO clay had darkened substantially. Luckily the tail was going to be covered with adornments but still, lesson learned.
Ideal Clay Mixing Scenario
If you're going to mix clays the easiest and worry-free way to do it is to mix clays from the same manufacturer so they have the same curing instructions, that way you won't have to worry about under or over backing one or the other.
If you still want to mix different clay manufacturers I would recommend curing in increments. Curing for 20-30 mins at a time and seeing how the clays are holding up in case your sculpture won't have the clay covered with adornments and it will be exposed.