The molex adapter will supply whatever power the card needs, up to the limit where it catches on fire anyway
Use the molex connector closest to the PSU to minimize electrical losses. As long as your molex cables are a large enough gauge to support the power draw, and the PSU can supply it on that rail, you'll have no problems at all.
The extra pins in the 8pin connector simply add another 12V wire to "spread" the current draw and avoid burning up wires.
As long as your adapter/molex aren't getting hot, you don't have to worry about that.