As I said earlier, Rufus can drop (old fashioned) insults but he isn't gonna vocally drop f-words or curses in gen unless he spend a lot of time with the likes of Elena,Reno & Rude & start picking up some of their speech patterns. Rufus has been educated in a certain way but also grew up in isolation & is still isolated. Children or people in general start to use that kind of words when they socialize with people or groups that already use those words or type of language. It's called mirroring. If someone like say Sephiroth or Genesis don't use those words, I expect it's the same with Rufus. We also never see him use the f- words canonically. His speech pattern tend be really analytical & sometime sarcastic, even in his mind in the novels.

Tseng won't openly drop it too bcos he's too professional. He might do it but it's also in his mind or when nobody is around.