How to get an old rusted plug out of some plumbing fitting?

Get a good torch: turbo acetylen, propane or close,
clear the area around the plug from any combustables,
cover the other surfaces; wet them down if you have do; keep water for
fire protection on hand ( garden hose from tap).

Heat up the female threaded part real good, apply you wrench (counter
clockwise!);
repeat if necessary but ensure that pipes are solidly fixed, use two
hammers and knock one outside of female thread part while using the
other (heavier hammer) as an anvil at 180 degree opposite,move around
to cover the circumference.
Heat up again( only the female threaded part)
Oil and penetrants only work after the sealants are burned out and
thread gaps are able to soaked in .

If not successful yet: get somebody stronger, buy a new house, burn
down the house if you got good insurance (not), blame the plumber who
put it together in the first place!