Organizers of the new All American Football League acknowledge the trail of failures. But theirs, they say, is a different approach: pro ball with a college sensibility. The league takes a first step onto the public stage today and Tuesday, days after the NFL pulled the plug on NFL Europa, when some 400 prospective players try out at the Citrus Bowl in Orlando. Additional tryouts are planned in September. Six or eight teams based in Florida, Alabama and other college football hotbeds, pooling graduated talent from nearby colleges and playing mostly by college rules, are scheduled for the AAFL's inaugural spring season next April. "Done right," says former NCAA President Cedric Dempsey, who heads the league's advisory board, "I think it's got an opportunity."