StarTech has been a reliable company for me on these types of things. To go "cheap" on such an adapter is not a wise thing to do, and the best ones are not expensive. I have not been able to see the images yet from Brian1965 (a forum moderator needs to review images and release them to protect our delicate eyes which is a bit of a hassle, but you never know what some toad might upload).
Here is the one I'd get, because at least on the 8-pin end all socket holes are populated. The pictures available of the adapter are good, but I can't look into the 6-pin end. While the HP 6-pin PCIe supplemental power cables provide more power than the ATX standard I'm sure HP would not have messed up the wiring order, and I've never heard of someone having trouble once they got this StarTech adapter in place:
StarTech PCI Express 6 pin to 8 pin Power Adapter Cable (PCIEX68ADAP)
Oh, on your other question..... those two 6 pin cables you have available can be used together or separate. Check out your video card max power draw and see whether you need one 6 pin-to-8 pin adapter, or an aggregator for 2 6-pin-to-1-8-pin adapter. Remember that you can look on the side of your Z620 power supply to see the total wattage that a single HP 6 pin cable can provide..... 18Amps of 12VDC..... I'll let you do the math. I'd not run that cable to its stated maximum amperage, for sure. The wattage through the HP 6-pin cable thus is quite a bit more than the ATX standard of 75 Watts. 8-pin PCIe supplemental cables from an ATX standard power supply can carry 150W... so you can see why HP feels comfortable providing the straight 6-pin to 8-pin adapter approach through their 6-pin cable. The PCIe slot itself will also feed 75 Watts up into the card from below, so take that into account in your calculations.