I think they are mixed. The white legs suggest old English game but not the white earlobes. The rooster has a modified pea comb, meaning it's somewhere in between a pea comb and a single comb. That also indicates mixed parentage.
Leghorn is a possibility for the other parent, but who knows...
I don't know who told you these breeds were easy to sex the first week, but they are not. Sometimes barred rocks can be sexed by the quality of their head spots, but this must be specifically bred for, and it usually isn't.
Hatcheries vent sex newborn chicks to tell gender of their chicks with...
Chances are he is just more discreet than the Cochin roosters. I've had many roosters like that.
If he is not the dominant rooster then he is probably not doing it where the other roosters can see it because they will attack him. Mating rights usually go by pecking order when there are...
I see no reason to believe the breeder was wrong at this point. Keep an eye on the combs for quick changes in comb size and color. That's what you're looking for in potential cockerels before 14 weeks or so.
Honestly, two of those chicks are darker than any Legbar I've ever seen. They could be hatchery quality or they might be a different breed or hybrid altogether.
I have a feeling they are not Legbars, and so it is difficult to say gender with those. They could be olive eggers, maybe.
The...
The one with pink legs could be, but the one with slate legs is probably a Phoenix bantam or a Dutch bantam. Hatcheries tend to sell bantam assortments to retail stores, so you can get anything unless they are in a bin marked as a specific breed.