The 6 pin PCIe power connector comes in a 6.25A (75W) version and a 12.5A (150W) version. The 6.25A version only requires that two 12V power connections be present, and only two ground connections need to carry current. The 3rd middle ground connection can be used to sense that the connection is plugged in. The 12.5A version uses 3 12V power wires and 3 ground wires to carry current. An additional two groundable sense connections can be added to the side to tell the PCIe device that a 12.5A compliant power connector has been inserted. Many power supplies that are 12.5A compliant have an additional 2 pin ground connector on the side of the 6 pin connector that can be used to make it to an 8 pin connector.
If a 12.5A 6-pin connector on an older power supply is connected to a newer 8-pin PCIe device, the PCIe device should not turn on because it doesn't know if a 12.5A compliant connection has been used. If all 6 wires are present on the 6 pin connector and the power supply can handle the current, then it is safe to get an adapter that will convert the 6 pin connector in to an 8 pin connector simply by grounding the two additional connections on the 8 pin connector.
The 2 sense connections on a 8 pin PCIe board could be permanently connected to the 6 pin ground connections with a little bit of solder and wire. This would convert it in to a 6 pin connection. Just be careful to not use any 6 pin PCIe power connectors that only have 2 12V wires, and thin wires that are smaller than AWG 20. If one fails to connect, all the current will go through the remaining wire and it could melt. If this is bad advice, then online sellers that sell Molex to 8 pin PCIe adapters should stop selling those first. That could easily end up runing everything through one wire.