Answers to questions
Sculpey is baked at 275F for 15 minutes per 1/4 inch of thickness. Fimo is baked at 265 for 20 minutes. The packages usually indicate the baking time and temp anyway.
When using sculpey in a larger scale you can use a wire mesh armature. My large orange hide and the blue long one with the snake on it were first shaped out of 1/2 inch wire mesh and the Sculpey applied on top and around the bottom edges. Cover the underside wherever there are sharp wire edges. Aluminum foil won't give rigidity to your structure, but you can use wads of it to help support tall sides or a domed hide while baking.
When marbling Sculpey I make a big ball of the various colors, roll it out into a sausage, twist it, roll back into a ball, etc., until I like the level of mixture. Then I take the ball and cut it in half. The interior of the ball has the best effect, because it hasn't been smugded by my fingers when working it. I flatten the ball with the cut side up trying not to smudge it again, and use it in my project. Also, the baked sculpey can be sanded like wood, and that way you can expose fresh colors and remove fingerprints. Polyurethane and epoxy will cover up fine sanding scratches and fingerprints as well. These coatings bring out the colors beautifully, especially with the metallic colors. There are quite a few books available on polymer clays that have fabulous ideas.
The two-part epoxy I use and have found at Ace Harware stores is called EnviroTex 1 to 1 Polymer Coating. You mix equal parts by volume and spread it or brush it onto your piece. It's gooey, sticky and messy, don't do it near carpet or upholstery because it might dribble on them. It behaves like thick honey, you apply it and come check on it in 20 minutes to find that it has dribbled all over the place. You don't feel it if it touches your skin as you work (it doesn't feel wet) and soon enough it's in your hair, face, toes, etc. I wipe it off myself and objects with paper towels. It has a workable time of 15 - 20 minutes and cures completely in 2 to 4 days, depending on ambient temperature. Practice with small items first to ge the feel for it. The results are worth the inconvenience.