"Giving someone the middle finger" originated in Ancient Greece and Rome, where the gesture simply implied sexual intercourse. Yes, it is historically accurate for our actors to flip each other off. I don't think they should though.
Rollerskating has been around since the 1780s, and the 1930s-50s in America was the Golden Age of Rollerskating!
If we assume Little Sally is counting pennies, the fee to pee at the top of the show is somewhere around five or six dollars. And that's at one of the cheapest amenities!