Re: Attaching limbs to a torso?
I assume that you want to cast the limbs separately and attach them afterwards. You don't mention the final material but there are a few options which should work with most materials.
If you want a removable fixing then neodymium magnets can be a good option if the parts slot together positively. Depending on the material they can either be glued on or you can get magnets with countersunk holes which can be screwed in place.
For permanent fixing a combination of adhesive and metal pins can be a good option for attaching limbs and similar. For smaller pieces you can make pins from short lengths of metal rod or wire, for larger heavier bits threaded rod can give a bit more strength to the joint.
Depending on your casting process it might also be worth embedding the pins into the mould itself.