Yes set it to 6 will ensure it works, the CPU will produce less work/ppd, but if you leave it @ 8 threads the GPU/GPU's will use some of the CPU cycles and slow the process down considerably
setting the CPU to run 6 cores leaves the GPU's 2 cores and both will run at the max
This produces lower CPU by about 25% as you would imagine, and the GPU's @ 100%
This is far more productive than allowing them all to fight for resources