Typically, a CR7 can depart ATL for Key West with no weight restriction. Obviously, this is based upon MEL and CDL items on the aircraft. I can't speak for the 737 though.

Departing Key West, we average 64-67 people. If we are carrying alternate fuel or have an MEL/CDL item that affects fuel burn, it becomes less. I've seen the payload reduced to 50 people on a CR7. On most days, the station requests volunteers to accept ground transportation to Miami. Again, I can't speak for the 737.

Long story short, your chances of getting to Key West from ATL are not bad as long as there are a few open seats. Keep in mind that we do have a few ASA pilots that commute to Key West and they have the opportunity to reserve the jumpseat 48 hours in advance. I would not recommend trying to leave Key West, though, unless the ATL weather is good and loads are light. A bus fare to MIA might be worth the cash instead of worrying about getting home.

Good luck!