Yes, excellent idea, it's easier to fit, more lasting and often cheaper.
Because you will then get a 'galvanic reaction'.
Yes, plastic pipe, PEX, can replace copper anywhere now.
unscrew it replace it, copper pipe coil easy to replace. Two pipe nuts about 3/8"
Detect the leak and replace that part of the pipe. It is usually better to replace the whole piece of pipe. By removing the junk plastic and replace it with quality piping such as copper DWV or NHCI
Galvanized pipe will be silver/gray color. Copper will be copper. Use a magnet, it'll stick to galvanized pipe but not to copper. Drinking water lines should not be black steel pipe.
That woulld depend on the piping materials and local codes as copper TYPE L , K or M should not be threaded and plastic piping should NEVER be lead wiped to a brass ferrule and Cast Iron should not be glued
When I have to connect copper to lead "D" I usually solder the copper to the lead or wipe a lead joint depending on the diameter To connect copper to cast iron ,.... If there is a hub I caulk the copper directly into the CI with oakum and poured lead (molten) OR one can solder / braze a female adapter fitting and screw the copper into a CI female fitting
Replace them with copper or plastic pipe.
If you had been advised why ask again?
1/2inch
If it is leaking at a fitting, it is either a bad flare or it isn't tight enough. Gas or flare fittings should be put together dry, Do no use pipe dope because this can cause a leak between the mating surfaces. If it is in the line itself, either replace the line or cut it, flare the ends and put a fitting in.
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