What is the exact power supply?
99 times out of 100, if your power supply doesn't have an eight-pin connector, you absolutely don't want to be using an adapter. The only situation in which there's a possibility that this is a reasonable idea would be two six-pins to one eight-pin adapter on an older power supply that was of excellent quality when it was purchased.
Using molex to PCIE, I wouldn't leave the PC running when I'm out of the room. And if this power supply is low quality, I'd make sure to have a sizable chunk of money on-hand to replace fried parts.