Will 1/4" NPSM products work with 1/4" NPT products?

Yes, all RAM® 1/4" NPSM flexible rod components will work with any RAM® female 1/4" NPT components. There will be some binding of the threads due to the nature of the NPT / NPSM threads. The mount will be secure when tightened as fully as possible.

How stiff is the flex of this pipe? Will it hold its bend if used with an action camera on a motorcycle at highway speeds? Also bumpy roads? Assuming a good secure mount in the first place, of course.

These pipes are very rigid, however if used on a motorcycle or in any high-vibration environments, the amount of leverage caused by the lengths of these pipes could cause excessive vibration at the ball & socket joints. With that in mind we do not recommend extensions such as these to be mounted on motorcycles or any extreme environments.