The film is based on a play by the great British playwright Peter Shaffer. Currently on stage again at the National Theatre here in London, btw, terrific production, well worth seeing if you can get here.
The premise of both play and film, that Salieri in his old age claimed that he had murdered Mozart, is perfectly true. It features in Beethoven's Conversation Books (books in which his visitors wrote items of news and gossip for him, since he was deaf). Nobody believed it; Salieri was a much loved elder statesman of the musical world, and everyone assumed he must have been going senile.
Several of Shaffer's plays touch on the theme of solid worth versus genius, the older plodding seeker after truth drawn to the younger source of inspiration.
Equus
and especially
The Royal Hunt of the Sun
.