There are one or two FNV creatures which would fit naturally into the Capital Wasteland, based on the approximate distribution of real-world animals: Wild Dogs, Coyotes, Mantises, Rats and Giant Rats, and the "pup" versions of any of the existing CW animals.
The bigger question though is what do we gain by doing so? Coyotes and Dogs would provide more sources of crafting items, Rats would be thematically appropriate but would need to be placed for maximum effect. Mantises would also need to be placed. Because of that, I'm not sure it's really worth the effort, at least given that it will alter the composition of the CW a little bit.
One thing that definitely needs to be addressed are Deathclaws and Radscorpions. CW Deathclaws and Radscorpions are a lot weaker than their Mojave counterparts, and as it stands at the moment most players will get slaughtered by early-placed Deathclaws and Radscorpions if they aren't tweaked. Radscorpions are a bit tricky, since small radscorpions are pathetically unthreatening, whereas medium ones are. We'll probably need to make a special version of the CW Radscorpion to compensate. As for Deathclaws, Young Deathclaws can be used, again with a possible adjustment to make them as threatening as the original CW versions. We oculd also just make a special version there. Either way, I think the CW versions need to be distinctly named, something like "Juvenile Radscorpion", that kind of thing, to make it clear that it's a different critter.
Now the small/young versions of other existing CW animals would make sense too, although again it would alter the environment a little bit. I like to think it would be for the better, but htat might be just my opinion.
One thing I can say that we should do is keep the altered death item/carry item lists for critters. Unless we want to complete suppress the NV crafting system in the CW, we need to keep those items, and those distributions.