I am looking to do the same thing. I only have chickens but I do have a friend who does exactly what you are wanting to do (as well as me). They have about 20 chickens, 2 guineas and 2 ducks (about 6 at one time but they unfortunately got killed). They all lived happily together. The chickens and guineas will roost happily together but typically the guineas like to roost the highest so there might be some compotition.
As for the ducks, I believe they sleep on the ground, or even outside in the run.
Speaking of run, you will need a wire roof over the top to keep the guineas from flying out (if you don't already). Also, if you buy adult guineas, you must be very careful if you plan on free ranging them. If they are not kept in the coop/run for months, when you go to let them out they will fly away and try to find their old home. You must be careful with them. But reprogramming them can be done!
As for feed, Nutrena All Flock (I think is what it's called) is what you will want, however, my friend feeds them normal chicken feed from the feed store, nothing special and they are thriving so it is up to you. If you plan on raising the guinea keets and ducklings from babies I recomend giving them the required food they need at such a young age to prevent them from dying on you while their immune system isn't fully developed.
I believe I covered all of your questions. Hope I've helped. And good luck. Reply back if you need anymore advice!
