Ok, so upon more research, it cannot be done with more than 4 controllers using XInput.
DirectInput must be implemented to allow for more than 4 controllers.
This is why games like Gang Beasts have the option to disable XInput and use DirectInput to allow 8 players to work.
I haven’t tested, nor have the ability to test, but perhaps 2 xbox wireless receivers could be used, with 4 controllers assigned to each of them, that might work, even though the receiver supports 8 xbox one wireless controls, Xinput has a hard limit of 4 physical controllers.
The reason a keyboard player and 4 controllers works, is that a keyboard does not really on Xinput or DirectInput.
The only way to get more than 4 controllers to work, is to implement DirectInput to the engine.
I may not do it right away as mini-games are the priority right now, but this is where I will be starting:
https://msdn.microsoft.com/en-us/library/windows/desktop/ee418273(v=vs.85).aspx
There are various plugins that have been created which implements DirectInput, but that means relying on those plugins to stay up to date with newer engine versions too.
I hope anybody reading this, won’t waste time chasing after a solution that doesn’t involve XInput.