I've got a SFF pc with a notebook DVD drive - it needs a floppy header to power it. The power supply I have for it has one more molex header than I need, and I have an old unused ATX power supply with a floppy header. I noticed that they basically use the same wires in the same configuration but with different pins, so would it be possible to take the floppy header from the old ATX power supply and use it to replace the spare molex header on the SFF power supply without cutting/soldering wires?
What you propose is quite reasonable, and you're right - floppy drives use exactly the same voltage supply lines as larger devices fed by Molex. There are two things to watch out for, though. First, how are your soldering skills? If you can't do the job well then fpsdominator has reason for caution. The other is wire size - the wires on a Molex are usually heavier, so there are both weight problems and size / ability to fit in considerations in putting a floppy connector on the end of 4 wires in place of a Molex.
The adapter from Molex to Floppy is an easy solution, and you should be able to find one at computer parts shops.