No, they are not metamorphic.
The novelization addresses pretty much all of your questions and theories. The kaiju are manufactured using cloned parts, but are each individual kaiju is not a clone itself.
"This was harvested in Manila, six years ago."
I killed the kaiju that gland came from, Herc thought. He stepped closer, crossing from Gottlieb's Prussian fantasy of order to Newt's intuitive maelstrom. He looked closer at the two glands.
They were identical.
Herc looked at Pentecost, who was looking at Newt with absolute concentration. In the background, Gottlieb was making a great show of ignoring Newt.
"Same exact DNA," Newt said. "Two different specimens, two exact organ clones."
"Same DNA," Pentecost echoed.
"Identical," Newt said. "Like spare parts in an assembly line. The entire organisms are obviously not the same, but different parts are absolutely taken from identical cloned snippets of DNA. This is a manufactured organ. It did not evolve this way. There is something more here than just monsters wandering through an interdimensional hole, and we need to know what."
"The kaiju... the reason I found identical DNA in two separate samples is because they're grown. Fabricated, assembled. Made of spare parts."
From the Research Report prepared by Dr. Newton Geiszler and Dr. Hermann Gottlieb
Dr. Newton Geiszler has conclusively demonstrated DNA repetition among different individual kaiju. Using multiple organs and a number of different individuals, Dr. Geiszler discovered repeated DNA markers in all specimens. These repetitions occur in the same sequences, suggesting three related possibilities
- The kaiju are manufactured
- Some of the repeated strands of DNA act as encoding mechanisms for a kind of species memory.
- The kaiju passing through the Breach transmit their experiences on Earth back to the Anteverse.
Dr. Hermann Gottleib endorses Dr. Geiszler's conclusions.
Kaiju science believes the kaiju are created weaponry. Given this troubling observation, it must be argued that we are in an arms race with the kaiju. The kaiju get larger and more powerful with successive generations (see attached chart plotting size against frequency of Breach openings).
We must confront the possibility that new kaiju will have built-in countermeasures to our standard combat protocols.
Finally, Newt contemplates the significance of Otachi being pregnant with a not-clone.
Seeing it was enough to make Newt rethink everything he thought he knew about kaiju procreation... and Precursor strategy. He'd known they had reproductive organs and assumed they could breed, but if a pregnant kaiju had been sent, and gone into combat first before trying to deliver its child...
Newt wouldn't have thought it possible for the news from the Anteverse to get worse, but he had a feeling it just had.
They wouldn't have to build every individual. All they had to do was hit on the right model and get two of them through the Breach to start breeding. If Hermann was right - a long shot, but always possible - that could start happening any time now. If four kaiju came through, and two of them could breed with each other, the other two could keep the Jaegers busy long enough that before anyone could do anything about it, the coastlines of Planet Earth would all be under siege by native-born kaiju.