Alex,
mix the pink fimo ( classical variant) with a little bit of black fimo.
Thus you will get grey.
By using good armatures one can make any shapes with it.
Polishing is extremely well done by soft brushing the object ( even without thinners or other acetates)
it's also cheap, in this way you will never spoil any of it.
It won't harden until baking.
when Fimo is baked, you can still carve if you like.
there are a lot of usefull things to know about thermoclay but basicly a big advantage is the lot of time left for altering.
Rene