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.

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