I keep Guinea, Geese, Ducks and Chickens together. And I know people who keep geese, peacocks, chickens together. I think as long as they have room to get away from each other if they want, you should be ok.
I do it, too, (guineas & chickens) but as far as the title of your thread goes, yes, I've heard that guineas and chickens really can and do 'mix species' with one another and the person who reported it (here on BYC) said it ain't a pretty sight. You might just mean can they live together, and, aside from that possibility, it's a yes for at least the two I know something about.