Use a rubber grommet in each hole or your car will be as bad as a 1970's Jaguar whose British designers thought a bare wire thru a sharp metal hole was OK. Many people just run the thick wires thru the plastic cavity where the ALT and BAT terminals were. It they melted the plastic, you don't even need to drill a hole.
Any size wire will work for a dash voltmeter. But, you still need the thick ALT and BAT wires to supply power to the cabin. The MAD Bypass just sends the large alternator charging current direct to the battery, not via the dash ammeter.