Is there a way to control six ftc tetrix motors with one arduino and motorshield.
Custom circuits shall not directly alter the power pathways between the ROBOT battery, PD Board, motor controllers, relays, motors, or other elements of the ROBOT control system (including the power pathways to other sensors or circuits).
You may be able to use the Arduino to control FTC type motors using the HiTechnic motor controllers. However, this product in intended to use the NXT to address the motors. If you can interface the Arduino to emulate an NXT controller then it should work just fine.
I do not believe the OP was planning on using this on an FRC Robot.
However make sure that you look at the datasheet for any motor controller sheild that you plan on using and ensure that it has the ability to provide the necessary current, there are a lot of cheap motor controllers on the Open Source market, so you just have to be a smart consumer.
Here is the datasheet for the Tetrix motor to get motor specs: https://c10645061.ssl.cf2.rackcdn.com/resources/dcdrivemotorspecs.pdf