Although this is "basically" a home repair Forum, there are experts here that do commercial work also. You really should post in the Plumbing section for more answers. Not sure what type "flexible" gas line you are referring to. There is flex line for underground use and coated flex line for hooking up appliances that is metal underneath. Most interior gas service lines are hardpiped.
If you have never made a mistake, you haven't done much.
Often called "trac pipe", the real name is CSST (corrugated stainless steel tubing). Last I knew, you need to take a little class and get a certification card before the supply house is permitted (by the manufacturers of the stuff, not the government) to sell you pipe and fittings.