I wouldn't risk it.
I had a house rabbit when my cat arrived as a kitten, and since then have had others, plus guineas as well, so she has grown up with prey animals around. The rabbits have been free range in the garden, but they've all been on the large side and since one didn't react to her trying to pounce on him, but, instead, ran after her she's left them alone.
I don't really trust her with the guineas, though. The only time I left her alone with the first group I had was when she was asleep / wanting into the same room as them for a snooze. But I'd keep the door ajar so I could hear when she jumped down from the bed.
She was ok with the group I had next, as they were very large, and wouldn't react to her so were of no interest. However, I now have two babies who are VERY active and do run away when she appears, and there is no way I would let her be in the same room as them unsupervised. Although she has hurt none of my other pets over the years I know that, if one of the babies was to move suddenly, her predatory instinct would kick in.
The fact that your cat shows an interest, and that the guineas react, makes me uneasy. I really, really wouldn't risk it.