Kyle, first, I'd suggest putting a cover on your pen. Guineas are strong flyers. Won't be long before they fly out of your enclosure. Another good reason to cover the pen is to keep predators out. Raccoons for example, will climb your fencing or trees and take what they want. Hawks, owls, etc. can just fly down for dinner.
If I read your post correctly, covering your outdoor area would be extremely difficult and expensive, so if I were in your shoes, I would enclose part of your space attached to the coop and use that for keeping them in at night. They can go in and out of the coop and into the fully enclosed pen at will if you want them to. Then during the day you can allow them into the uncovered fenced area for free-ranging (they will eventually fly out of it).
Chickens only need to be confined for a few+ days or so. Not so with guineas. They need about 6 weeks to get the idea. Some people think more than six weeks. If you want to put them outside to start your "6 weeks" confinement, you can add a heat lamp to the coop for night temps. It won't hurt the chickens...If you can, just run a few electrical cords (for indoor/outdoor use) to the coop and plug it in at night. But that depends on how far away your coop is from an electrical outlet.

If you can't do that, I'd keep doing what you're doing then when they're old enough, put them in the coop/pen for six weeks. But know they're going to get out sooner or later. Try to make sure they know where home is before you let them out.
People have differing opinions on free-ranging guineas 24/7. I'm an advocate of penning/cooping at night. It keeps them safer and it keeps them closer to home, and they develop a routine that's easy for me to monitor.