Borderlands 2 – Beginner’s Guide, Episode 2 – Modes, Scaling, DLCs, OP
TL;DR: On Normal, Scarlett’s and Torgue’s campaigns are supposed to be taken alongside the main quest, at any point since level 15. Hammerlock’s and Tina’s – after the main quest. On TVHM and UVHM – any campaign at any point.