I wouldn't use styrofoam as a filler as it can off gas in your oven and may damage the sculpey by doing so or cause you and anyone in your home health issue. I like to use aluminum foil as its inert to sculpey and doesn't rapidly expand in the oven which is key to not having a cracked sculpture.

For a freestanding sculpture use the heaviest gauge wire that you are able to work with for the main supports; things like limbs and the core of the body. If it's a small sculpture I would then bulk it out with aluminum foil to keep your clay costs down. If it's large you need to epoxy all the joints where you've twined the wire for you armature and may need to pre bulk with epoxy.