In the reference circuit from this datasheet for a potentiometer, what is RL for? Why did they pick 100 x R? Bonus question, how do I pick a good value for C?
It isn't "for" anything. The manufacturer is simply saying that in order to get the rated performance out of the potentiometer, whatever load is connected to the wiper should be greater than 100× the value of the potentiometer.
The capacitor, in conjunction with the effective source resistance of the potentiometer, creates a low-pass filter. Since the source resistance varies with the wiper position, the cutoff frequency varies accordingly. The maximum source resistance occurs at the center of the range, with a value of 1/4 the potentiometer value (assuming both ends are connected to voltage sources), and it decreases toward either end. Select the capacitor value so that the cutoff frequency stays in the range you need.
It is probably serving as a pulldown resistor to the capacitor. these are quite common at least in the circuits I've seen.