Miles, most never get near salt and a lot of the fancies are very salt sensitive but they are widespread and rivers end in seas, What I have been seeing is that some tolerate lightly brackish water. I sort of doubt it is really good for them though.
The Hypostomus ventromaculatus and this one (pictured) Ancistrus gymnorhynchus have also proven to be fairly salt tolerant.
I know for a fact the regular plecos are living and breeding in south florida in brackish canals. As to the level of salinity, not sure, but it's salty to the taste.