There aren’t, they have never been established in the manga and there’s literally never been any proof for them
Yes they are since Orange Town actually.
Captain is usually the strongest, not always, then we have the second strongest, sometimes a duo like at Kuro crew, and then the third, the forth etc.
Buggy crew we have. Buggy. Cabaji. Mohji.
Kurt crew we have. Kuro. The cat brothers. The Michael Jackson illusionist.
Arlong we have. Arlong. Kuroobi. Hatchan. Chew.
Crocodile is even more explicit since we have their numbers. Crocodile. Mr 1. Mr 2. Then I would say Mr 4 is actually stronger than Mr 3.