The only issue with the hypothetical scenario you describe is that these two rails are also tied together with three cross braces as well as being tied together with the gantry. Assuming the guide rail cars have negligible play, the force would be distributed across both side rails.
I absolutely explored building something much smaller. My findings were that the cost did not go down as significantly as I had hoped. My initial plan for the current design was to be able work 1/2 sheets of 4x8 foot material. I entertained going with 1/4 that size, 2 feet x 2 feet, and the cost was roughly half, as my cost for structure is effectively zero.
I may still try something closer to 1 meter by 1 meter.
Want to say how much I appreciate your feedback. This particular aspect of manufacturing is new to me and I love a challenge.
Thank you
Guy