Is it safe to recharge a water fire extinguisher yourself?
I'm just guessing that you are referring to drain, waste, and vent (DWV) piping which operates under little or no pressure. Normally, waste and drain piping is made up from slip nut connections to simplify access for routine maintenance. In my experience, teflon ferrules will positively seal joints with finger tightness (if the piping and ferrule are clean). Ferrules (nylon or other plastic) are used on PVC and brass waste and drain piping. Rubber is used in some fittings but the performance depends on the integrity of the slip nut rather than the pipe wall (usually it is inferior).
No you cannot use pvc, cpvc or poly for a gas line. Gas piping should performed by qualified people who then pressure test the system for leaks.