Know the situation, know the hitter. If the hitter is a dead fastball hitter, a first pitch Slider will throw off his timing and may mess with his head.
Since he is a fastball hitter, he most likely will take the first pitch Slider. Getting you ahead 0-1 in the count.
If you can throw a good Slider that looks like a fastball and the hitter commits to swing, you will make him look downright foolish.
If the hitter is a free swinger, even more so, a first pitch swinger, throw the Slider and see how disciplined he is at the plate. An undisciplined hitter will either get himself out or get himself behind in the count. Use it to your advantage.
Best Of Luck