For instance, has every AA 757 visited all destinations it serves with a 757? Or are different groups of them sanctioned off in to different geological areas?
Yes and no. You picked perhaps the absolute worst AA
fleet type for commonality so the definitive answer for the AA
B757 type is NOT A CHANCE!
Only a few AA
757s are equipped for high-altitude airport ops. A few more are equipped for ETOPS and even more are equipped for "overwater ops." All can be sent to any domestic [read: North America] destination, but outside of that aircraft availability quickly goes down.
OTOH, the AA
737 fleet is [currently] totally common with all aircraft [attempted to be] maintained in the same configuration. This month's schedule realignment is [partly] designed to take advantage of this fact by having more 737s assigned to domestic [business] routes previously flown by 757s and more 757s assigned to the limited 737 international routes.