I've de-coupled the arduino and pi power circuit using a 47uF electrolytic and two 100nf ceramic capacitors.
I'm not even sure what this means, since both Arduino and r.Pi board certainly have their own capacitors
on-board already. So, ???
where would I place the electrolytic capacitor on the arduino? Or should I run the 5v onto a circuit and do everything there, just relying on the arduino to read the results?
You should add said capacitors out at the power feed to the servos, since that is where the motor noise
is being generated.
You have a good system here, in that you have both 5V and 3.3V controllers. You just need to be sure
to connect the 5V extra components [sensors, sonars, etc] to the Arduino and the 3V devices to the
r.Pi. Then, all you need are 3 wires TX,RX,gnd going between the 2 controller bds, plus a level-shifter as
G_M mentioned on the TX pin from the r.Pi.