What will stop the customer from plugging in a full size welder is that the breaker will trip. That's what breakers are for.
If you install #6 wire, you will use a 30 amp breaker to protect the welder. You can't fuse equipment rated for 20 amps at 50 amps. If someone plugs in a full size welder, the breaker will trip.
The comparison is a range plug. They are rated for 50 amps and are commonly wired with #8 (45 amps) and fused at 40 amps. If someone plugs in a larger range, the breaker trips.
