What do you think about me telling this elderly couple to fuck off?
I hope you're joking. Why would you tell an elderly couple to fuck off?
They're yelling and screaming like crazy and ruining the movie
It's annoying when others make noise in a movie, but you don't have to curse at them to get your point across. Try asking them politely if they can be a little quieter.
Screw that, I going to tell them to fuck off.
Seriously I understand that you're upset but telling them to "fuck off" is likely going to just make things worse. If you calmly get your point across, you'll have the rest of the theater there to back you up as well and they'll get the idea even faster. Cursing at them will likely just escalate the situation.