Fountain pumps aren't really designed to handle the significant back pressure created by misting nozzles. They're really only made to push water through a tube that's mostly open with perhaps only a sort of fan spray attachment or other decorative feature. Most of the cheaper ones are really only designed to move water from the reservoir to the top of the fountain.
As you can imagine, water doesn't necessarily like to be sprayed into an ultra fine mist, so you have to use extremely small aperatures. Basically, you have to force water into a hose that only slightly leaks.
Conversely, a fountain pump is designed to push water into a hose that only slightly restricts the output. It may work, but for how long? If you have to buy a $40 pump two or three times, you haven't saved any money but you have dramatically increased the hassles in your life.
You're better off getting something made to do the job and just buying it once.