Although very fine tracks can be etched to avoid surcharges its best to treat 8 thou as a minimum track width. 

The same 8 thou guide also applies to the clearance gaps, but I would advise increasing up to more like 15 thou on earth planes. This is mainly to guard against the extra risk of solder shorts when the boards are assembled. With a tight clearance on a ground plane it is also normally very sensible to request a 0 Thou clearance photo imagable solder mask.

Be warned even though foreign several electronic web sites specify 2.6mm clearance for mains voltages , in the UK a least a 4mm clearance is usually expected for mains voltages to comply with our low voltage directive. Even then on some boards we are asked to rout out air gaps to increase the isolation.

Should you require an air gap its simple add an extra layer to your design and use it to indicate where the router paths are required.