We have a 9 year old cat that we introduced to the ducks when they were almost full sized. We introduced them VERY carefully, with ducks in a pen, just in case. The cat is kinda fat, and lazy, and not a hunter and when she discovered that the ducks were sometimes involved with WATER she decided that she wanted no part of them. Now we let them in the yard together, and they're fine. Sometimes the ducks follow the cat around.
A tiny kitten showed up in the woods a couple weeks ago. We introduced him to the ducks too. They are much bigger than him, and so far he's afraid of them.
However, he ACTS like a hunter. I always watch him carefully around the ducks, because even a playful scratch could develop into an infection. And NO WAY would I let him in the room where the chicks are, and I put him away before I open the hospital cage. I wouldn't trust the fat cat with the chicks either, for that matter.
Once the chicks get bigger ... I don't know. I have a feeling when they are full sized, the fat cat will ignore them also. But the kitten will be bigger too by then, and I'm not sure whether I will ever be able to trust him around the chickens, or the ducks in future either. I guess it really depends on the cat, as far as I am guessing.
I'm hoping I can make him trustworthy with them. We also have gerbils, and he goes crazy playing with them through the glass. One accidentally escaped a few days ago, and I found them playing together on top of the gerbil cage. Maybe he's really just playing gently enough. Or maybe he hadn't gotten around to biting yet. Either way, nothing happened before I found them, because the gerbil would have bitten back, and both were fine.
So I guess I have to wait and see.
Somehow I think it must be a lot easier to train dogs. Dogs I am used to. Cats I am never sure about.