What do those two things have to do with the number of pitches a pitcher throws? Those two things are absolutely true. They also are unrelated to throwing 6 pitches. It's not somehow more stress on your arm if you throw 1 sinker, 1 cutter, 1 4-seam FB, and 1 slider compared to 4 sliders. Generally I'd expect it to be worse for your arm that you threw 4 sliders.