Oh, then you would definitely use a plaster mold. UltraCal-30 is typically referred to as the finest plaster for molding, but you can get a away with anything. There aren't any hard rules.
This is a great book that covers almost every topic: https://www.amazon.com/Stop-Motion-Skills-Animation-Effects/dp/0240520556
I would also recommend using plasticine over sculpey if you're going to make a mold. The heat created as the plaster sets could harden the sculpey and make things a lot more difficult.
Tom brierton has a pretty good book called Stop motion armature machining if your interested in ball and socket armatures. I have a ton of books on the subject but can't for the life of me remember the titles. give me like an hr and I'll see if I can find them and I'll add them as an edit
Edit: Ok so here are some that I used to learn that I found very useful before I got into the industry.
Stop Motion: Craft Skills for Model Animation (I have the first edition of this book but I'm sure the second is even better) https://www.amazon.com/Stop-Motion-Skills-Animation-Effects/dp/0240520556
The Art Of Stop Motion Animation. (I met the author of this book at the Ottawa animation festival a few times and its definitely a good read) https://www.amazon.com/Art-Stop-Motion-Animation-Ken-Priebe/dp/1598632442
Creating 3D animation (This was the first stop motion book i ever bought and its got a ton of great information and pictures) https://www.amazon.com/Creating-3-D-Animation-Peter-Lord/dp/0810919966
I know there are more but I cant find them at the moment. Ill keep looking around and I will update if I find them. Additionally feel free to PM me if you have any questions on ball and socket armature machining. I spent a lot of my time in stop motion doing armature work and would be glad to help if your having trouble with any specific aspects. Armatures can be pretty complex and when it comes to documentation its highly limited. Good Luck!
EDIT 2: Also books that I highly recommend though you might not think it are "The art of" and :Behind the scenes" books from major stop motion animation movies. The Nightmare Before Christmas one is particularly good and the Coraline one is excellent. With these books though they don't teach you so much as you can find little gems hidden in them. Look closely at the pics and you can see armature construction drawings with sizes for all the parts and diagrams of armatures and weird ways of doing things that you wouldn't expect but that tend to work great (thats usually why Laika uses them :)