you've got to unlock the cooperstown teams by getting the team card itself. then you'll be able to unlock the individual players.
Sorry this just isn't true. The historic players are going to be made available in future downloadable content. So far only Series #2 which is the traded players is available.
Custom sliders are analyzed and given a R, P, A, or L rating for Rookie, Pro, All-Star, and Legend (respectively). This is used for card challenges and achievements.
I've heard reports that wild cards aren't awarded in custom difficulties, but I haven't verified this myself. But, even if this is the case normal cards and challenges are awarded.
Custom sliders are analyzed and given a R, P, A, or L rating for Rookie, Pro, All-Star, and Legend (respectively). This is used for card challenges and achievements.
I've heard reports that wild cards aren't awarded in custom difficulties, but I haven't verified this myself. But, even if this is the case normal cards and challenges are awarded.