Cutting aluminium will be fine regardless of which screw you use since it's only possible to use around 0.6-1.2m/min. With regards to resolution 5mm is better, but it's not a big difference and you're unlikely to notice it. If the critical speed is 500rpm, then yes that means you can get 5m/min, but not 7m/min since that would be exceeding the critical speed. The critical speed formula should be treated as a guidline, since it doesn't take into account the support from the ballnut and relies on a constant you select for the end-fixity, which is a bit of a guess. However I still wouldn't be happy with operating much above what the formula states, even if it looks OK. Have you seen irving's motor calculation spreadsheet on this forum - it may help clarify things.