You're fine. It's just that anyone with any experience at this jumps past the series resistor method so quickly it is a blur, especially if they want variable control. As I said, it is very inefficient. But it works just fine if you pay attention to the power levels. The pot you linked to will work with no problems, but it probably will not take the fan down to 50% of full speed. It will make a noticeable difference in both air flow and acoustic noise. BTW, you never said why you want to do this. It could change the kinds of answers you get.
ak
ak
