Your local may or may not allow copper pipes; it depends on what additives
they put in the gas. Mine allows thick walled copper.

No place allows soldering. Some locals allow only flare, some allow flare
or compression. I don't know why; if solder is good enough for high
pressure water it ought to work for low pressure gas, but maybe solder just
isn't stable with gas.

Check with your town; they don't have code requirements for fun.