the mission my players are on right now are pitting them against a large number of devil rats and a number of naga amongst other things. They're curious whether either would provide reagents for making foci, lodge materials, etc. Anyone have any experience or suggestions on this? I'm less inclined for the devil rats and in a toss up for the naga.

Thanks!

With a sweep of his...

Hat

Entirely up to the discretion of the gamemaster, but generally speaking you'd have to determine if any of the critters had a suitable part for a reagent on them (a devil rat's tail with seven kinks, a perfect fang from a naga, etc.) and then make a Gathering test.

Then again, you should differentiate between reagent and telesma. The nagas at least might make some interesting telesma, even if they provide no reagents.

Is there something somewhere that makes using the parts of sapient beings as telesma/reagents a no-no?

Not explicitly, no, and dragons of course are well-known as both sapient critters and potent telesma.

<Ghostwalker>: You want me to do "what" in that cup?

This thread reminds me of a couple D&D games I've been in. As an example, some "traditional" dragon parts for magic might be:

teeth - spartae warriors (Ancient Greece, Jason and the Argonauts)
claws - fetish
scales - armour
blood - potion material, poison, bathe in it so that the skin cannot be pierced by any weapon
eyes - all seeing magics, crystal ball component, etc.
bones - rods, staves, wands, big ugly club, jewelery, fashion accessories other stuff
bile - potion material
etc.

There are plenty of parts to choose from from many creatures. Just look at some creative uses of critters in the past, and what parts are used in "traditional" medicine today as a starting point.

teeth and claws i would peg for weapon-foci O.o

Tiger testicles for various magical medicines are necessary.

Those uses are covered in Augmentation.