If I dont have raw files available from a shoot, but do have access to high quality JPGs or TIFs, I assume it is fine to import JPGs and LR will still maintain quality files? It seems that Lightroom will treat the JPGs like a RAW files and be non-destructive?
That is correct...Lightroom treats al files as nondestructive... so to see the results in other apps a new file with the adjustments mad in Lightroom needs to be created through an export....
Yes- Import all JPGs and TIFFs.
Lightroom itself will not over-write or damage the 'original' image file that you import. LR edits are only in the catalog (until you 'Export' a derivative copy!)
But-! it is possible to open the original files in a 'Pixel' based editor (Photoshop, etc) and force them to over-write the 'original' - so beware when you edit outside of Lightroom.