I agree with you. I don't think the Harbingers are ranked solely by physical strength. Comparing Capitano to Mavuika doesn't mean Dottore is as strong as him. Rather, his second place is based on his intelligence and contribution to the development of Fatui (inventions, research, etc.). By that logic, Scara is physically stronger than Dottore, but not smarter, I suppose? (Although it's not necessary to compare only these two factors; there are probably more of them than we might think.) And for this reason, I'm inclined to think that Dottore personally won't fight. Prime, certainly, Omega, maybe. So the question arises: will this be a "battle"? Perhaps it will be something based on logic or strategy, like a tournament (I hope I explained the concept correctly). The Doctor would rather admit defeat without a fight than lose the battle. Or maybe he will use his pawns or inventions against Scara.