"can i power the arduino uno with 5 volts on the Vin header pin?"Yes.Put a 1N4001 or comparable diode from +5 (cathode) to Vin (anode) to protect from reverse biasing the regulator."6-20 are the limits and 7-12 is recommended" at the barrel jack or at the Vin pin.
Vin pin can only take up +5v....correct?(this is by passing the +5v regulator?)
so i bought two 5 volt wall wart power supplies