My experience with adult guineas and chickens is that it depends on the type of chickens. My adult guineas will NOT stay penned, they fly out of anything not covered. The chickens I had were Austrolorps and they were too heavy to fly out of a 8 foot pen, however my banties would fly the coop along with the guineas as soon as the sun started rising. Hope that helps.
Thank you! That's is exactly what I was looking for and hoped to hear. The only bantams I have are a few silkies, cute but not bright so I doubt they are going anywhere. Not that I don't wish freedom on my chickens but we have a serious number of predators and to be totally honest, a wrap around deck, poop free and staying that way!