I have never had Guineas so honestly, I can't say. What I have heard is that they are great at sounding the alarm. It only stands to reason that a bird (other than one with long sharp talons, maybe) is not equipped to fight a raccoon. Your best bet is to have your ducks locked up at night in a raccoon proof shelter. Make sure there is nothing like garbage, dog or cat or duck food around to draw raccoons, even a bird feeder will entice them. If you you feel you need a guardian animal, a large protective dog would be a better choice. Of course, then you need to TRAIN it to stay in YOUR yard, not eat your ducks, or your visitors and provide all the care and maintenance a dog needs.