not true at all, it COMPLETLY depends on the drakes personality. i know so many people who keep just a pair (1 male and 1 female) who do perfect together.Let's talk about your plans. You only want 2 Ducks? 1 Hen and 1 Drake, I assume? That's not a good idea. A drake Duck need a haram around. Having only one mating partner is really bad for the Hen. He will hurt her, badly, come mating season.
Consider keeping 3 Hens and 1 Drake. I think that's why the breeder sent you 6 instead of 2.
i have two pekin drakes who can absolutely never be with any hens, they are 3 years old and can only be with each other and/or with my other drake. my khaki drake is good with his two girls, he’d be fine with just one girl too. all depends on the individual drake.