I think you re trying to learn too much thing in the same time. I mean modeling, ncolth, fur, rig, render... First you should make a perfect model or later you ll have more problem. If your wireframe is no good, your rig wont work the way you want
Try to post some wireframe screen & I'm sure this community ll help you. 4 example why the arm not rounded or how you should change to get a better result!