I disagree, Williams would be a good pick anywhere in the draft (provided he stays healthy). Remember Norv Turner is his Offensive Coordinator, he has a history of making RBs into stars (i.e. Emmit Smith, Terry Allen, Stephen Davis, Ladainian Tomlinson). Miami is going to give Ricky the ball about 400 times this year.
You might have shot at Green at 5, but not Alexander. I would take a RB with the first pick, no doubt.