To be honest I would say that a careful keeper who does water changes with some regularity should be able to keep 3 to 5 bluegills in a 75, I doubt they'd get to be 12" more likely 6 to 8 inches but they would still be nice fish and if feed and cared for should show some really good colors.
Again IMHO choosing the right fish is the key, choose a sunfish species that has a small maximum size, you can keep more of them and a more varied species list. A tank with madtoms, blue spots, black bands, top minnows, mollies, or if you prefer a predator then gambusia and redfin pickerel can be added to the tank when the sunfish get large. makes for a very interesting display and doesn't give people the impression they have to have a public aquarium class tank to keep native fishes...