might be more economical to make your own intelligent motor controller with an atmega32u4 breakout board
and a handful of l293d
's. if you run out of pins on the microcontroller you can throw in a couple of 595
's and rig a super motor shield.
have a look at the motor shield schematic to see how to wire it all up.
the mega32u4 breakout has enough pwm outputs for 6 motors and enough a/d pins to sense their positions, but probably not enough pins left over for directional control for the motors, which is where the 74hct595s would come in. for 6 motors, you'll need two. the clock, latch, and enable inputs would be the same for both 595's, but pin 9 of the frist one would connect to the serial input, (DIR_SER) of the second one.
"If I had known it was harmless, I would have killed it myself." - Phillip K. Dick, A Scanner Darkly