I have the same motherboards & have no issue running 13 GPUs on Linux. You can get it working on Windows but Linux(nvOC, PiMP, Ubuntu) you plug them in & immediately mine with any combination really. I use all Nvidia cards though. The 8+5 builds are all based on the limitation of Windows.
nvOC has a UI so it's easy for those not comfortable in the terminal. I highly suggest taking the 10-15 minutes it'd take to download & set it up. I love tinkering with my rigs, but I never need to, ever