Alright so i had some more help outside, just for future reference i'll describe what was wrong, and what solved it.
First off, the power supplly was indeed insufficient, so i'm adding a 2nd one, this should solve the cards failing.
Besides that, i had a monitor hooked up to internal graphics, this seemed to cause problems, so instead i'm now using a gpu for the display, which made one more card work.
With that fixed i can manage to get all 6 cards working!