Regardless of the past, I doubt it was the plec. I'm pretty sure the gourami was not well. Also, what do you mean by "I bought him a buddy?" Male dwarves are very competitive and aggressive. If two males were in the tank, the one might have killed the other.
, there are parasitical catfish that do feast on fish scales and gills, but they're not Loricariids, like your pleco. Conversely, there are also cichlids that feed on catfish scales!