Background Myth: (see the Dragon and its Nine Offsprings)
A Dragon dying and splitting up its essence into 9 parts, giving birth to 9 'offsprings'- each a Kaiju different in appearance from the normal Dragon. These offsprings each have their own territories but they could fight with each other since devouring their siblings is one way to make themselves stronger.
This Dragon is part of the Original Creation and its death means that the Basic Elements that are part of the Dragon will separate without the Dragon as the "central glue" to hold them together.
These Basic Elements might seek out compatible lifeforms to bond with OR be accidentally/coincidentally 'swallowed' by existing lifeforms with the result that some sort of 'mutation' occurs, giving rise to Kaiju 'offsprings' of the Dragon.
Ideas about appearance of the Kaijus:
A giant insect kaiju. Or a giant mole with a drill on its head. (Maybe you want at least one of them that can burrow?) You've already got the water serpent. I feel like one of them should be hooved, so maybe the kirin or else the forest god thing in Princess Mononoke?
Your current list seems pretty mammalian. Giant turtle, a la Gamera? What about a plant, a la Biollante? Or have the Formless Dragon, which was formed last from the leftovers and is a blob that can form any other parts. Maybe the Formless Dragon is bitter and jealous of its better formed siblings.
I know that some of this stuff doesn't seem very traditional, but some good writing could spin the story so that it all makes sense.
And why not inject some foreign mythologies so that your dragons are less Chinese remixes and more weird? The Thunderbird from Native American mythologies. Sleipnir from Norse. A giraffe neck from Africa. Sobek from Egyptian. Camazotz from Mayan.
You might get some inspiration from the Tailed Beasts in Naruto http://naruto.wikia.com/wiki/Tailed_beasts, since they're a bunch of giant animal kaiju with a common, mystical origin.
From Caesar: -Komodo Dragon, but instead of four legs, you get a bunch of centipede legs.
-Frog with mermaid tail
-A horned rat. The horns have poison.
submission 1- a mountainous continent that is shaped by the fight b/w two of these Kaijus whose territories border each other: their footsteps create valleys, they kill each other and their spines become the two tallest mountain ranges, their decaying bodies enrich the soil etc.
Submission 2- a Kaiju that has its domain in a submerged continent, a cross b/w a water serpent and a swordfish (or maybe a unicorn is more appropriate, basically, it has a horn sticking out of its forehead). Its milk feeds its children but also the intelligent races living there.