About the belt - I wouldn't use it except when you're going for really big lifts. You'll probably be the best judge of when to use it - if you don't feel as comfortable with the weight you're attempting, you may want to use it.
As for the arm work, I do a fullbody routine 3x per week. I do 3 sets of direct bicep work (BB curls) and 3 sets of direct tricep work (tri extensions) per week. To me, that's enough, because the compound movements I'm doing stimulate those muscles heavily.
But you know your body and your goals, so you have to adjust what you do based on that. If you think you can stand a bit more volume than that, you can add more sets. It all depends on your goals and your body.