There is no way via scripting to set a document's rulerUnits (pts, mm, inch, etc) because the property Document.rulerUnits is—erroneously, surely—read only.

If Document.rulerUnits was writable, we would simply do this:

  var doc = app.activeDocument;
  doc.rulerUnits = RulerUnits.MILLIMETERS;

This is important because there is literally no way to set this via script, except to create a new document.