Short-tails and K'Chain Nah'ruk (KCNR) are two names for the same thing. They were the evolutionary ancestors of the KCCM, and went extinct. They were later resurrected by the Matrons (the implication being that the KCCM had genetic manipulation technology), used as slaves, and eventually rebelled. This history is from Kallor's narration in MoI, so trust it as much as you trust him, but I don't see how it would aid him to have lied about these details. Of course, he might just be plain wrong in the first place.
Karsa fought and killed a lone KCNR in the keep in BH, and he did it in style.
Ganath was ambushed and killed by KCNR at the end of BH (she reveals to the reader that they are aspected to order, and the KCCM are chaos-linked, as she lies dying).
Which of the two races inhabit the sky keeps seen in the Imperial Warren is unknown. I think Cotillion says they are KCCM, but we're not sure if he even knows that the KCNR exist, he may just be assuming that "sky keeps = KCCM" because he doesn't know any better. And no-one looked inside.
The reason I keep insisting the two companions from the RG prologue are not KCCM/NR is simple. Red Mask appears to be of a humanoid race. I seriously doubt that any KCCM or KCNR would ever travel with or even talk to members of another race, probably a vanilla human. All the evidence that we've seen of the reptilian races suggests that they simply fight everything and everyone. They don't talk, or interact in any other way, with other races.
Caveats - the KCCM we've seen so far have been dead (MT), undead (MoI, the K'ell Hunters) or insane (Pannion's Matron).
We've got two examples of KCNR behaviour, and they are
a) fighting Karsa, getting killed
fighting Ganath, winning swiftly.
No opportunity for much dialogue in either case there. But I really really don't think we'll see the KCNR use dialogue of deal with other characters. They're alien, genuinely so. They're not like the other races, which are pretty much just humans with different bodies. I think of them as dinosaurs.