I use a 12v power supply (3S lipo). I drive 2 stepper dirvers, 3x Ultrasonic, 2x IR sensor and 1x XBee (directly via 3.3V). I may add more later. Till now the 800ma max (http://playground.arduino.cc/Main/ArduinoPinCurrentLimitations) should be more than enough. --> in theory I have three clocked step down supplies lying around.
From your point of view, does it make sense to:
1) Keep everything as it is (most simple) 2) Step down 12V to 7V and keep supplying everything via the arduino 3) Step down 12V to 5V supply and supply the sensors and stepper drivers from this supply. Keep the 12V supply for the arduino. 4) Better idea?