There was one of the sillier Showa era Godzillas (Godzilla vs. Gigan?) Where Godzilla does briefly talk. “Come on Anguirus!” He yells as the two swim to fight the evil monsters. Maybe it was only in the English dub?
Those three rules are awfully fluid.
It’s not really eating if he spits instead of swallows (Godzilla Minus One).
He can’t talk…unless it is to other monsters using his own language (Ghidorah, the Three-Headed Monster, All Monsters Attack and Godzilla Vs Gigan)
.. And he can’t truly die…unless he does (Godzilla 1954) and they just name a new monster of the same species “Godzilla” (Godzilla Raids Again).
Next Toho film Godzilla Minus Zero coming next year.
In Godzilla, King of the Monsters, didn’t he eat and swallow one of Ghidora’s heads (after nuclear-flame-roasting it)?
They aren’t particularly fluid, they just weren’t implemented until 2019 or thereabouts.