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No, Not Dominiques or BR X Dom Crosses, Dominiques have rose combs and rose combes are dominant so if they were BR X Dom, they would have rose comes too.
There is no research or rule saying Black Sexlinks have yellow leggs though hatchery crossed sexlinks do becuause they are made using two yellow legged breeds. But say if you were to cross a Buff Orp with Cuckoo Marans hens, then you would would get white legged Black Sexlinks.
Also there is no set age at which roosters are supposed to grow. 5-7 months is the norm but I had one go to at least 9 months before he started crowing.
Like magicpigeon said, in a group of roosters that are together, ones the first ones start to mature it can put a suppression on the other lesser roosters and cause them to be submissive and more timid and develope slower. Becuase of this I usually separte at breeding roosters by them selves with hens when they are starting to mature so they can bloom out and know that they are the boss of that pen without having to worry about other more dominant roosters.