It all depends on the dog. Rescues take in adult GSDs from shelters every day. These dogs can and do adjust to new situations. It is true that some dogs have issues. Some are easily correctable. Some can only be managed. As Jelpy suggested, you would really want to have the shelter GSD evaluated, before even considering her. The dog bit someone. Not knowing the circumstances, it's hard to say, but many would not take the risk. Unless there was a very clear reason - such as the dog had been hit by a car and was in excruciating pain when it bit, a rescue won't even take on a dog with a bite history.
Natty Boh 6-27-12
Annie 1998 - 8-2-12 RIP