I've had guineas and chickens live happily together now for about 9 months. 4 hens, a roo, and two mating pairs of guineas. That said, one of my guinea cocks has just utterly lost his head for mating season and become HUGELY aggressive - beating up my rooster terribly. They're actually flying through the air at each other, trashing, drawing blood and pulling feathers (this started in the last couple of days). Sadly, I think I'm going to have to cull the guinea.
A mistake I've made, which caused this problem: I had the guineas and chickens in the same coop, and they chose to lay in the same spot. They're all free-range during the day, even in the winter, so exercise or crowding are not to blame - and the coop itself is 10'X6', which they use only for laying and for the night. The fighting happens during the day, then they all gather in a bunch to sleep. Odd.
At any rate, I've really enjoyed the mixture thus far, and they've gotten along really well until now. Lots of space and freedom during the day is a must, and if you can, try to get the guineas to lay their eggs somewhere other than in the same box as the chickens.