Water constantly exerts pressure on pipe joints. I have seen old soldered joints flying apart when subjected to extreme pressure or trains passing nearby. This sometimes happens with plastic pipe also if joints are not thoroughly set .

Pipe dope and/or teflon tape

No, Qwest pipe and joints are OK with well water

Pipe size refers to the diameter of the pipe (in inch).

A pipe joint is a key component to any successful plumbing system. Pipe joints means the connection at the ends of pipes that ensures tight sealing and strength. In some cases, pipe joints must also provide for rapid assembly and dis-assembly or a change in the direction of the line of pipe. The most common types used for general-purpose metal pipes are welded, flanged, threaded, and bell-and-spigot joints. alliedallcityinc.com