you won't kill a fish by giving it fresh water, you might cure it though. Anyone who thinks a 95% water change will kill a properly cared for sick fish is living in the 80's or so as far as their fishkeeping goes.
If one would 'properly' care for a fish... chances are lower that it would not get sick to begin with. But regardless when a fish is sick it is weaker and should not be put through extreme condition changes.
And yes according to the cat-e-log sailfins in general get bigger than common plecos, go search for species info on planetcatfish on the species names I gave in my post above. If your info is better than that site on plecos then email them and get them to change their database info on size.
Planetcatfish has been known to have wrong sizes for plecos... there are many other sites out there. But I like basing things on experience. PC has a P.Pardalis at 13.8 inches. I personally have had two of them over 17 inches. The biggest pleco I have seen with my own eyes was a Pardalis which was about 22-23 inches (it was a monster).
PC has a sailfin at 17.7 inch and I have seen a 20 inch one before.