Blacl Molly Completely Vanished

One of my Black Mollys some time ago had some problems, like her back was
broken and she was swimming sideways and all around. Someone mentioned a
teaspoon full of salt, which I added and it did no good. It actually had
appeared her back was broken some way. I went on vacation over thanksgiving
and when I came back, she was gone. I decided to look and removed all the
possible hiding plaes in the tank and looked and the fish is no where to be
found, totally vanished. Our LFS owner told me that my Pleco, which is
pretty big now, about 8 inches, would start eating my other fish. Is this a
possibility/probability? The Pleco seems totally docile and hasnt messed
with any of the tetras or the clowns.