Hello there @KadenKitsune! I would recommend this tutorial for your demo, as it covers the whole animation and rigging process really well. The channel also has other videos on FPS creation topics, a very good resource.
Regarding multiple arms, and setting them up for enemies, as long as you tie your animations to a main skeletal mesh, you can duplicate and re-use the same skeleton for other NPCs. Just make sure that their general shape and scaling are similar.
Finally, when working on your duplicates, and creating enemy variants with different shapes or ammount of limbs, you may need to re-target your animations. If this the case, please follow this guide on how to perform the process. Hope this helps!