voltage supply I would either use a 12V supply + step-down converter to 6V or a 6V supply + step-up to 12V.
Half the voltage is twice the current (same power).
Sorry for too many questions, Is this good to go ?
Since you didn't tell us what the motors should do, it's hard to say if a solution is good enough.
I haven't seen any replies addressing the fact that the OP plans to power the UNO (or Pro-Mini) from the same battery powering all the motors. I do see an issue there with the motor noise generated and it's effect on the uP. Motor spikes on the UNO regulator input could result in errors or resets unless you add some substantial filter caps (470 uF or greater).
Don't know.Wait and see what happens.