Thanks for the confirmation. The servos may or may not be used in conjunction with the motors but, yes, I plan to use a BEC to power them if I do mix and match. The balance plug of the Lipo is what I had in mind to connect the Arduino to so that the main motor supply can be turned on separately.
Fancy suggesting that anyone would connect an external power supply to the 5V output pins ! Who would do such a thing ?
Confession - me in a mad moment of not thinking. I can confirm that the USB section of the Uno does not work any more and that the main chip no longer works in another Arduino, but it may be recoverable.
That could be a dangerous practice as that would be forcing a unbalanced condition between the individual lipo cells and could cause damage when full current is drawn by the motor from all the cells. And while a balanced charger would/could correct the unbalance condition on the next charging, it doesn't prevent damage from unbalanced condition during the discharging of the pack.