Cornies are rated to 125 I believe, and have a pop-off valve that will blow before the keg as a safety mechanism.
Carbonators do run at 100psi to get the warm water carbonated fast.
To get soda level carbonation you need 30-35psi in a keg of soda or water at a temp of the low 30's. It can take a couple days just standing upright to dissolve all the CO2, laying it down speeds things up a little, shaking it can reduce the time to under 30 minutes.
I don't know what pressure the fountains run for the soda water at the fountain head, but imagine is is MUCH lower than the 100psi at the carbonator. If you figure out the pressures, a tank of soda water should run a fountain just fine.