If there is short in the cable between the pod and the shore power, the shore power breaker should trip (regardless if it is a 30 or 50 amp). The 50 amp breaker should require a little more short circuit current to trip than the 30 amp but it should go on a direct short. The magnetic pick up/trip on these breakers is typically 10-20 times the handle rating of the breaker. That is the level of currect required to trip the breakers magnetically. The only thing that you would have to worry about would be an overload when running on the 50 amp circuit. This being said, the 30 amp main breaker in the pod should protect against any overloads that are within the pod as they are downstream of the pod main breaker.
I have had cases where the 30 amp shore power breaker trips for no reason (and I have to hook up to the 50 amp breaker) and others where the pod main 30 amp trips (due to AC fighting with a hair dryer!) and the shore power breaker remained on.
The thermal elements (what protects against an overload condition) in these breakers are made of bimetal or trimetal type materials and are "calibrated" at the factories. These calibrations can vary quite a bit. You may get a breaker with a very "short" calibration or one with a fairly "long" calibration. It is a crap shoot to be honest as to which breaker you may get.
Due to the fact that we have a 30 amp main breaker in the pod's to protect against shorts and overloads in the pod's themselves, I have zero issues connecting to a 50 amp shore power connection knowing that if there is a short in my cable, the shore power breaker will trip.
Todd