I'm running a dnd Hollow Knight campaign online so I've been making some HD versions of some of the enemies, here are two mosskin

There is of course not much. What we have is mostly environmental and the following bits:

"The Mantis tribe and the bugs of old Hallownest had no love for each other."

Seer: "The folk of my tribe were born from a light. Light similar to Essence, similar to that powerful blade, though much brighter still. They were content to bask in that light and honoured it… for a time. But another light appeared in our world… A wyrm that took the form of a king. How fickle my ancestors must have been. They forsook the light that spawned them. Turned their backs to it… Forgot it even."

Quirrel on the Spider Tribe: "Supposedly, there's a village deep in the warren. Its inhabitants never accepted Hallownest's King."

The Fungal Tribe: "Warily shall we accept the will of the Wyrm. Its prescience shields us. Fate and Future we shall entwine."

Mosskin: "Though once our lands, a pale being lays claim to the caverns ahead. [...] Be wary to wander that place."

It really is only the archive and Fog Bugs under Monomon that the King's reign seems more or less accepted happily (based on the elegy she wrote) and of course the other bugs of the other regions who formed the bulk of Hallownest as a kingdom. All of the rest of these seem somewhere between fully antagonistic or at least only reluctantly accepting that the Pale King is inevitable.

As for what the Pale King did: I think its great that Team Cherry didn't explain, because it means I get to make up stuff based on this lore to fill hundreds of pages of fanfic about it! The wider the holes the better I eat.

Personally, I think that the Pale King aligned with the simple insects and moths after chasing off the radiance and becoming the new light of Hallownest. After that it probably depended. I mean he did have an army of armored bugs and five powerful knights. I'm sure they waged war. In fact, the Mantis' alude to making a "truce" with the payment being their independence for keeping the Mantis Door closed and beating back the beasts of Deepnest.

However towards the end, before the Infection truly began, there is some evidence that PK was going around that truce and forming an alliance with Deepnest in the form of birthing Hornet, accepting silk from the Weavers (seen at the stag station near the White Palace) and building two tramways (Though in the collapse one failed and was overtaken by Deepnest again).

There's also the clear implication that some Mantids did not accept the truce. The Traitor Lord abandons the tribe with a contingent of traitors to explicitly take back the Queen's Gardens and kill the White Lady seemingly in revenge for the love his child shared with a knight of Hallownest.

𝓝𝓸 𝓶𝓸𝓻𝓮 𝓰𝓲𝓻𝓵 𝓫𝓸𝓼𝓼.𝓘 𝔀𝓪𝓷𝓽 𝓽𝓸 𝓰𝓲𝓻𝓵 𝓻𝓮𝓼𝓽.𝓖𝓲𝓻𝓵 𝓼𝓵𝓮𝓮𝓹. 𝓖𝓲𝓻𝓵 𝓵𝓪𝔂 𝓭𝓸𝔀𝓷 𝓸𝓷 𝓽𝓱𝓮 𝓯𝓸𝓻𝓮𝓼𝓽 𝓯𝓵𝓸𝓸𝓻 𝓪𝓷𝓭 𝓫𝓮 𝓪𝓫𝓼𝓸𝓻𝓫𝓮𝓭 𝓫𝔂 𝓷𝓪𝓽𝓾𝓻𝓮.

𝓘 𝔀𝓪𝓷𝓽 𝓽𝓸 𝓰𝓲𝓻𝓵 𝓶𝓸𝓼𝓼

Pssst on the topic of new little guys, he isn’t exactly new but this is my boy Nugget :> I play him in a Hollow Knight prequel dnd campaign and he’s a mosskin who ended up landing a single hit on a Mantis Lord and they said “damn. You’re pretty dope for a little guy. Take some Soul and go wild out there.” and as such he’s just kinda, wandering in hopes of becoming powerful enough to be a knight to some great entity.