Yes, my brahma bantam keeps my red stars pretty fertile. He has two four week old red star-brahma crosses running around out there now to prove it. These are 2 of the four that hatched.
Note if it was sebright roo, I don't know if he could fertilize, but he would certainly try. What breed is the rooster? (Reason I ask is there's a big difference in size between my bantam brahma roo and my sebright hen).
What about OEG roos and Bantam Cochin hens, and vice versa? I'm thinking of moving my OEGs and Bantam Cochins into a seperate coop and run from my egg layers. I don't really want them to cross breed, but don't have but one other coop to move them to.