Inside Guys
How did the inside guy of each pair get their jobs. Martin a lounge singer and the rest porters or other low level staff. The Sammy Davis character already worked as a trash collector so that fits. Seeing how the plan is revealed a few days before the job all five casinos needed to hire and immediately put to work one of the guys without a background check. Maybe there was high turn over in porters but how likely would the Dean Martin character been booked on the spot for New Years.
Hoping for a better answer than "it's just a movie don't think too hard about it."