Well it didn’t really make sense that the Japanese scientist knew Godzilla would take care of the other monsters.
Godzilla is a surrogate for America.
The big overwhelming force that came out of the sea and devastated Tokyo.
Americans came and pretty much flattened Japan from a distance. They they arrived in person, big, loud, scary, uncouth.
The Japanese (quite reasonably) expected these loud invaders to rape, pillage and burn, to demand draconian (hmmm, like a dragon!) reparations, to bleed Japan dry and discard the husk.
That was the first Godzilla movie. It fit the temper of the times, it was an attempt to psychologically cope with a new world turned upside down, a world where Japan wasn't ascendant.
Then it happened. The Pale Destroyers (hat tip Blue Lancer) didn't destroy Japan! They helped Japan rebuild everything they bombed! Rebuilt the infrastructure, established a constitutional government, established entire industries, became the biggest customer, and fast friend and defender of Japan.
Huh?
HUH???
The following Godzilla movies attempt to grapple with this conundrum. Godzilla vs this that and the other creature. Godzilla not quite as a friend of Japan, but an least as a fellow combatant against other threats to the Home Islands.
And then Baby Godzilla was downright cute!
That's the what the Japanese scientist knew from his youth. The big scarey monster turned out to be not so bad after all.