1 unit = 1 mm. Small scale will get you loads of problems later ...
There is nothing wrong with this. You can use whatever scale you want. The problems arise (if I'm not mistaken) when working with very large numbers because you started with too small of a scale to begin with. But if your program is based on small objects in a small environment then there is no problem with using a scale such as 1 unit = 1 mm.