The Arduino can run on 5V at the +5V pin or on 7-12V on the Vin pin. The servo is probably designed to run at 6V so neither choice is ideal. Perhaps you could use a hefty 7+V supply and put a few big silicon diodes in the line going to power the servos. A silicon diode has a voltage drop of 0.6 to 0.7V so add drops to bring the voltage down to about 6V.