The Skinnies are allies of the Bugs in Heinlein's Starship Troopers. What do we know about them and their alliance with the Arachnids?
According to Juan Rico, the Skinnies are humanoid, eight or nine feet tall, much skinnier than humans are and have a higher body temperature. And they don't wear clothes.
These geezers are humanoid, eight or nine feet tall, much skinnier than we are and with a higher body temperature; they don't wear any clothes and they stand out in a set of snoopers like a neon sign. They look still funnier in daylight with your bare eyes but I would rather fight them than the arachnids — those Bugs make me queezy. (Ch 1)
The Skinnies were allied with the Bugs but changed sides and allied with the Terran Federation in the middle of the war.
The novel was told mostly from the point of view of an individual way down the Mobile Infantry chain of command; for tactical reason he knew nothing about intelligence matters or high level strategy. So the novel has nothing to say about why the Skinnies allied with the Bugs or what the nature of that alliance was.
What a man doesn't know he can't spill if he is captured; neither drugs, nor torture, nor brainwash, nor endless lack of sleep can squeeze out a secret he doesn't possess. So we were told only what we had to know for tactical purposes.
I mostly agree with @Kyle's answer. We don't know for sure.
However, there some basis for informed speculation.
One seeming reason that Skinnies changed their alliance and allied with humans was that humans - via Juan Rico and his jumpy-happy-bombing-squad-action - demonstrated to the Skinnies just how much pain humans could bring.
Nor did we know that strenuous efforts were being made to subvert the alliance against us and bring the Skinnies over to our side; the nearest we came to being told about that was when we got instructions, before the raid in which Flores was killed, to go easy on the Skinnies, destroy as much property as possible but to kill inhabitants only when unavoidable.
Therefore, it's quite possible that the original alliance with Bug may have been due to exactly same reason - Skinnies allied with the biggest (only?) bully on the block, as the alternative was getting stomped on, permanent like.