If I have a G-sync monitor, should I enable Fast Sync as well?

Fast Sync is unrelated to G-SYNC, but they work together to provide the best gaming experience. Fast Sync should be enabled when games have very fast render rates relative to the refresh rate of your monitor. G-SYNC is most effective when the render rate of a game is variable, and below the max refresh rate of your monitor. So both technologies can be active at the same time to deal with variable game frame rate scenarios.