If you look at the PCB of Enermax, it is not problem, but who knows

The rest of boards I will test with Fortron AURUM PRO 1200W 80PLUS GOLD (one single 100A 12V rail). Just for sure.
Great, & you can try to use 9(1+2 disable), 3(2+3 disable), 5(1+3 disable), 6(0+4 disable)
sorehammer & y_boonstra already confirm that multi rail on one system is fine so maybe the problem is the board