According to Ralph, his family's discipline IS for everybody. My wife beats me up all the time. The whole point to this style is to take the opponent down and restrict their mobility in the mount or guard. Realistically, odds are in a street confrontation, the grappler will come out on top. It is too hard for a stand up fighter to prevent being taken down to the ground.
Some guys can pick it up faster than others.
You don't have to be athletic. Although five belts may not seem like a lot to accomplish, it's not about how many belts you can obtain, but how hard you work for them. Advancing in the Gracie's system puts dedication and effort to the test. This emphasis is in putting the opponent in what is called the guard. No one is better at working the guard than the Gracies.