hello,
Nice work.. however, if i was you, i would put all the motor in the base arm (the shoulder) and find out a way of transmitting power to the rest of the robot through wires. I did that last year, with bicycle brakes metallic wires, and it worked perfectly, we had a robot with movements controlled by 6 motors, and they were all in the base of the robot. This way you reduce inertia, also you need less power to move the 'end effector' of your arm.
good luck and keep us posted