I don't know if this is discussed already or not, but I would like to share my theory about the Red Hair Pirates.
My theory about the Red Hair Pirates is that they don't have any Devil Fruit users in their group! It seems to me that they want to rely on natural powers, such as Haki and speed. Let me explain:
Shanks greatly idolizes his mentor and the First Mate of the Roger Pirates Silvers Rayleigh, who is one of the strongest pirates ever and doesn't use Devil Fruit powers. He is very fast and can use all three Haki perfectly. He's also a swordsman so Shanks wanted to be a swordsman too. Shanks also clearly didn't care much about the Devil Fruit what they found back when he and Buggy were apprentices in Roger's group.
He saved Luffy from the Sea Monster with tremendous speed and Haki (and being able to swim!), and his crewmate Lucky Roo was fast enough to sneak up on one of the bandits without being noticed. Red Hair Pirates also stopped Kaido attacking Whitebeard before the Marineford war, and after that managed to come and end the war which the Marines thought was impossible.
So there! Red Hair Pirates use only natural powers (speed, Haki etc.) and want to be able to move quickly from one place to another and every member has to be able to swim to save whoever they want to save.