What PC make an model do you have?
If its a dell PC you can only use a Dell MB. The PSU is proprietary, and I believe other connections might be as well.
I have used 6to 8 pin connector to GPU and also on a different one have used sata to 6 or 8 pin as well with no issues.
Just make sure your PSU can support the power from the GPU and CPU together
I still have my Dell 7050MT, but I found it was limited for what I want to do.
It has a decent I5-7600 CPU and 24gb ram, but PSU is too small to support a midrange or better GPU.
So I build a new PC from scratch.
I still have the Dell, but plan to build another PC, then Ill give away or sell the Dell.