I think you got some bad information...I AM an engineer, and my recollection and understanding of current - is that it travels only along the surface of a conductor. So stranded wire offers allot more surface area for current flow than does the same gauge solid wire conductor.
Also - if workability is your concern...I've had luck with running the heavy gauge wire from the power source to a set of terminals or bus bars and the final... say 10-20 feet, that has the most bends perhaps to the charge controller, could be stepped down in guage - so in your case, you'd run 6ga to a final and SHORT length of 8ga...you will likely NOT be able to measure your loss nor heat diff...