I have donated a few tiny Dutch bantams and a bantam RIR to several chicken owners who had standard commercial laying hybrids. I saw /heard they got along well immediately or very soon after arriving.
I didnt keep in touch with the new owners. So I don’t know if the tiny /small roosters could fertilise the hens. The people who took my cockerels, mainly wanted a rooster for protection (alarm) and fun. And a small rooster is easier to handle. Also, Dutch bantams roosters look stunning with their large tails.
I have not fully integrate him with my flock, but I have let them mingle for an hour in the afternoon.
Yesterday he broke up a fight between the 2 bantam hens and he pecked at one of them. I did not intervene.
This afternoon, 2 standard size hens were fighting each other, he stood in between them to break up their fight, but he only stand to the middle of their necks, they were pecking at each other on the heads while he was in the middle using his body to push one away and so the other, he was not able to break them up. I intervened.
They know he is the rooster for certain, there are 9 standard size hens and they did not pick on him. He is tiny, but brave little rooster, he stands watch while the hens hangout, and he run over whenever he hears one scream.
He did jumped on our biggest hen - Plymouth Rock hen Apple, his body length is half of her body length, we took him off her so I don't think he managed his deed, but even if he did, he won't be able to fertilize her..I am guessing.
I still have much to learn on this.
