It can import ADS *.slm file format and can export Asitic format. Asitic export wasn't tested much, I just implemented it for my personal use. It you decide to try, feedback is appreciated. Good luck!
The cell size in the ASITIC example tech file is even larger (2µm), but of course it depends what geometry dimensions you will create/simulate.
As mentioned above, this ASITIC output from m/matl wasn't tested much, so no warranties of any kind that it works for your needs
There is no "safe" value, the cell size determines how accurate the solver can place current/charge onto your metals. It really depends on your geometry details, and finer cell size allows more accurate modelling of high edge currents etc.
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That said, 1µm is a typical value that often works, but if you create a balun with 5µm trace width then you need a finer mesh (smaller cell size).