Don't overthink this.
If a card has a 6pin and an 8 pin plug on it, it is clearing the way to grab 150 watts from that plug and 75W from the 6 pin. Mac isn't wired to provide 225 watts through it's 2 plugs.
If a card requires an 8 pin, avoid it or clock it WAY down if you want to use adapters.
The assignment of which plug is on a card is not random, it is there for a reason.