Hi,
I have a similar situation. However I would like to use 2 batteries.
1 to power the motors/servos
1 to power pi + arduino
After reading the specs, it seems to me that my best bet would be to power the arduino using a 2 or 3 cell lipo giving me 7.4/11.1 V which is within the allowed range for the arduino. Then wire a min usb plug to the arduino +5 and gnd pins to power the pi.
I'm not an electronics-pert so was wondering if this would work without breaking anything ?