I think that some other regulars at this forum use a barn, but I don’t. I’m sure that they would have some great insights. I would just consider a few questions, like: 1) How safe is it? Can a raccoon get in? A weasel? A snake? 2) Can the guineas get in easily enough that you can fence it off or otherwise teach them to use the access door that you want? 3) How hot does it get in the summer? Is there ventilation?
The problem with your plan is ,though guineas do fly, their normal mode of travel is on their feet. They are walkers, their food is on the ground and that is where they spend 99% of their time. They walk in and out of my coop and although I built a trap door above their flyway expecting them to use it. They don't, only when I have had adolescent birds who do tend to fly more than adults .did they occasionally use it when they got freaked out about something and they would go in. Guineas do like to roost high but their food and water should be ground level. If you do nothing for them ground level with food and water and shelter, they will most likely just end up roosting in the trees at night. And guineas that roost in trees don't live long. There is a lot of information on this site about coops. Good luck!
