Would it be a possible solution to mix vendors?
It definitely is a viable solution, except implementing it will be far from trivial.
Turn off any unused device in the BIOS for should you run out of resources at the BIOS level, there is nothing the OS can do - some GPUs will remain uninitialized / unmapped into memory.
Far easier to use two systems with 4 cards each.
Would you agree there is a distinct possibility that boards with multiple GPU slots may have been designed with maximum flexibility in mind?