I think that the size of the beveler should be at most a third of the thickness of the leather. If you choose a beveler too small the effect is not visible. If you choose a beveler too big you cut a v-shape and you can not round the edge. For example if you have 4mm thick leather I would recommend a beveler of at most 1.3 or 1.5mm.
No. Absolutely not. I am sorry that this is not the answer you wished for. But it would look truly terrible. Do not cheap out. Go for Palosanto bevelers and purchase 2-3 different sizes. My recommendation would be 0, 1 and 2. 0 for card holders, 1 for portfolios etc and 2 for belts in approx 3.0 - 3.5mm thickness.