I don't think there's a way to set it by rule, it's a fixed property of the primitive. At best, you can only have it check and throw errors on primitives out of spec.
After the fact, you can query,
IsTrack OR IsArc OR (..other shapes..) AND (Width < 6)
(using AsMil or AsMM as needed if you switch modes frequently), and assign a new Width to everything in the Inspector. But you'll have to unlock everything, too, which includes primitives in footprints. PITA.
Tim