It makes no sense why use of a small-scale nuclear weapon would somehow escalate to armageddon. It seems to hinge on the assumption that government leaders are

It makes no sense why use of a small-scale nuclear weapon would somehow escalate to armageddon. It seems to hinge on the assumption that government leaders are completely irrational robots that, once a nuclear weapon of a size equivalent to a conventional bomb is used, take that as license to use nuclear weapons of any size. It's much more likely the "unwritten rule" you speak of would just shift, where nuclear weapons of a certain payload (probably equivalent to conventional weapons) would be acceptable for use. Government leaders, being acutely aware of MAD, would clearly communicate the new accepted boundaries and proceed from there.