Yes they will work for what you're asking for. I had 2 of them here and to be honest they were to good as a watch dog. During the day they never would stop yelling. I had just 2, "I" would never want 8.
Some things about them. If you pen them up for 45 days or so, they will NEVER leave your (their) home, that is if they are young. If you get older ones they could leave and go back to where they came from, so it would be best to luck up adults for 90 days or more. I have heard story's of adult Guineas traveling 100+ mile to get back to the yard they came from. If you are lucky they will roost in a tree, if you are not lucky they will roost above your bedroom window, point is they will roost where they want and you just can't stop them from being in that spot. When they sleep, they are "no" good as a watch dog. You can walk right up to them pick them up and bounce them in your hand and they will not move/wake up, why that is I have no clue, just hope they roost high because a bad animal will get them at night. When you pen them up, but the pen where you would like them to be the most, because they will stay in that area and start to slowly open up their living area little by little once you let them out of the pen. I am sure there is more to tell you, but right now that is the most I can think of. Oh my Guineas are no longer here. What happen do you ask? We gave them to a friend 15 miles away. He penned them up for a week, lol they were back here on their own around 24hrs after he let them out. So he took them back and penned them up for 45 days, they haven't been back and just last week they are alive and well driving him crazy with their noise.