flex vs rigid electrical conduits
 

I have a warehouse area that has alumimum electrical outlet boxes
mounted against the walls and connected to each other via rigid
alumimum conduits. All voltages are 120V.

I am thinking of extending the circuit from one of these outlet boxes
by using flexible alumimum conduits to another wall that needs some
outlets there. Can I use flexible conduits or do I have to go with
rigid? If flex is OK, is there any code that specifies how big the
conduit must be? I plan to run only 1 circuit through the conduit.

To mimimize work, I plan to purchase flex conduits from Home Depot that
already come with 12/3 wirings in them.

This is just a storage warehouse that is not exposed to any moisture
and etc. We also don't really draw much power from these outlets.

This is in the LA, CA area.

Thanks