When you are trying to model arms it's important to think about where our arms bend and move. You have movement at the shoulder, elbow and wrist. Everything else should be kept as straight as possible. Work the fondant to narrow the wrist between the hand and arm. Use your fingers to pinch in at the back of the elbow to give it a little more of a point. If you don't pay attention to the little details the arm ends up looking like a wet noodle! Believe me I know from experience
Take a good look at your own arm in the mirror and use that as a guide. I attempt to keep my figures as much in proportian as I can but I can not scuplt a realistic face so I do keep mine "cartooney". I like them that way. I feel that if I can't do it well enough to make it really look exactly like the person I'm better off to just make it in their likeness and emphasize their features.
Good luck with your project.