Is temporarily reducing the water supply line an option when installing a water softener?

Yes, it will reduce the water flow.

How do I connect my water softener to the main line?

Quote from the video:
Quote from Youtube video: And we connect it with a 3 8 compression. Fitting. We make a connection at the softener. And also at the brine. Tank. Connect the drain line to the drain elbow and we secure that with a hose. Clamp.