The cats are 21 months, so they are well past the kitten phase. They were raised as barn cats to hunt mice and were allowed in the house as pets only since this winter so they have a strong hunting instinct. There has never been any problem with the bantam, but once or twice I saw Grochatila look like he was hesitant about whether she was a bird.
I have now seen how the bantam mother reacts when the rooster comes near and it is clear she will defend them from anything and scream at the top of her lungs. This should be enough to make him hesitate and alert us. However I agree it's too early for an introduction, we'll wait till they have grown a bit.