Would the plot of The Sting have worked if the character of Salino had never been introduced? While she does kill one of Lonnegan's other assassins, she doesn't appear to impact the plot in any other manner. Yet her character, while a surprise during the reveal, seems to lack substance, at least to me. Was her character necessary?
No, the character of Salino is not at all crucial to the plot of The Sting.
Rather, Salino murdering the competing hitman, Hooker having a brief affair with a "random" waitress, and then Hooker subsequently nearly being assassinated by her, are all merely colorful sub-plots that - though consistent with the main plot - in no way advance the main plot. The "affair" part adds color / helps build Hooker's characterization / injects a little romance into the story, while the fact that various hitmen are trying to "knock off" Hooker serves to maintain the tension throughout the story. The final "reveal" that Salino is, in fact, the waitress is a nice "twist."
The removal of Salino / the waitress from the story would in no way damage its integrity or result in plot-holes, but the movie would, as a whole, be less interesting and thrilling.