There are two things here: Rivalry and power levels.
Rivalry is between two characters, we see plenty of examples in Mangas: Rukawa v Sakuragi, Rukawa v Sendo/Sawakita, Naruto v Sasuke, Sasuke v Rock Lee, Goku v Vegeta, Goku v Piccolo etc.
Half of my examples are rivalry between people with the same power level, the rest are between highly different power levels. I would spit on anyone saying that Rukawa and Sakuragi are not rivals, even if one is the best player in Japan (arguably) and the other is a novice.
Buggy and Shanks are slightly depicted as rivals, and the story pushes them a bit in that direction, but it's not even comparable to the levels I describe above.
Going back to Zoro v Sanji, this is a rivalry about being "better" one of the other. They both make remarks about each other, (1000 v 2000 men worth, lately the comment of Sanji during Zoro v Lucci). I don't give a shit who's stronger, Zoro is canonically stronger than Sanji as his role in the crew enstablishes it, but this doesn't mean that they are not "rivals".
Rivalry is between two characters, we see plenty of examples in Mangas: Rukawa v Sakuragi, Rukawa v Sendo/Sawakita, Naruto v Sasuke, Sasuke v Rock Lee, Goku v Vegeta, Goku v Piccolo etc.
Half of my examples are rivalry between people with the same power level, the rest are between highly different power levels. I would spit on anyone saying that Rukawa and Sakuragi are not rivals, even if one is the best player in Japan (arguably) and the other is a novice.
Buggy and Shanks are slightly depicted as rivals, and the story pushes them a bit in that direction, but it's not even comparable to the levels I describe above.
Going back to Zoro v Sanji, this is a rivalry about being "better" one of the other. They both make remarks about each other, (1000 v 2000 men worth, lately the comment of Sanji during Zoro v Lucci). I don't give a shit who's stronger, Zoro is canonically stronger than Sanji as his role in the crew enstablishes it, but this doesn't mean that they are not "rivals".
I somewhat disagree since ''rivalry'' in Shonen is usually about who is stronger but if the above is your definition than why not ! That's not the case for most people though; for almost everyone rivalry = around the same ''strength level''