I usually separate the parts after the armature has been fleshed out, if I plan to mold the piece when done. As has been mentioned, a little forethought is necessary and the figure should be planned out with the idea that you will separate the parts. Once separated, I then attach the pieces as I work on them. Perhaps I'll finish the torso, clothe it, then I'll attach an arm and clothe it so that it blends naturally with the finished torso and also has a clean separation joint. I'll lay on a little petroleum jelly to the finished surface of the torso to prevent the new material on the arm from sticking and allow the arm (or whatever piece you happen to be working) to pop loose once it's cured.
If you're working entirely in polymer clay, I've seen sculptors separate after completion (before baking) by using a fine wire to cut the pieces apart. Your armature will be a problem though and it will need to be cut through as well, so again, planning and forethought are important. I've used this method with mixed results on smaller work areas.
For scaling, I find it extremely helpful to have a commercial figure in the desired scale handy to use as a gauge for scale comparison. I pick one that is particularly nice and which has good, even anatomy.
I am currently using Super Sculpey for my faces and Magic Sculpt for everything else. I live alone and do most of my cooking on the stove top or the microwave, so I'm not concerned about odors and such in my oven, figure parts are the only thing I bake in it.
For heat curing Magic Sculpt (which, as has been mentioned previously, will cure just fine on its own, the heat simply accelerates the cure time), I made a small "oven" out of a cardboard box, which I lined with aluminum foil, and a simple light socket and chord w/plug I bought at Home Depot. I've used it for nearly fifteen years and all I've had to do is reline it occasionally and put in a new bulb when it burns out. It's been all over the country with me.