Will this hose work for both coolant and boost lines?

Yes. The 3-ply silicone handles both applications. Just match the pressure rating of your system to what this hose is rated for—verify the PSI spec before installing on boost circuits.

Do I need hose clamps, or does this seal on its own?

Use stainless steel hose clamps on both ends. Clamps are not included—get ones that fit your barbed fittings or hose barbs.

Can I use this hose if my original tubing was a different diameter?

That's exactly what this reducer does. As long as your intake or return line is one end and your component is the other, this steps the diameter down smoothly. Verify both connection points before ordering.