Not true. Magikarp turning into Gyarados is a reference to an old Chinese legend.

I dont see it, the only throughline is colour.

It doesnt make a lot of sense for two stages of dragonites line to be all gentle and serene, only to end up angry and violent.

Whereas the helpless magikarp becoming the terrifing and powerful gyarados has narrative symmetry.

I thought the same about bitterfree and venomoth. Just look at them. The eyes, “mouth” and everything just looks switched. But apparently, like gyarados and dragonite, they weren’t actually switched either.

No.