The Sebrights will take the longest for you to know for sure because the cockerels/roosters are hen feathered (meaning they will have the same plumage as the pullets/hen)
You're correct on breeds!
So far, nothing's screaming cockerel. I'd think the d'Uccle would be showing signs by now. However, your Sebright will take a little more time before we can be 100% sure on gender.