I have a total of 8, three roos, two hens, and 3 babies. They do try and bully the chickens somewhat, but only when it's feeding time. I feed mine the same as I feed my chickens, which is chick starter (unmedicated), flock raiser, and cracked corn. They also free range all day, and they (the adults) have decided to roost in the trees.
Mine don't make too much noise, and they also don't wander too far. They usually stay closer to the barn than the chickens do, who are always in the woods. I read that about 90% of their diet is bugs, so that is #1 reason why I got mine. We had a huge tick problem, and haven't seen 1 for about a month. I have only had my guineas for about 6 weeks, so that's pretty good!
Ask the1much or Miss Prissy about eating guineas. I know the1much has, and pretty sure Miss Prissy was the 1 who told him how to fix them. As for me, mine are just watch dogs, and pest controllers. Plus they are pretty funny to watch.
I would say you would probably need two of them, since they are a bit wilder, and have different habits than chickens do. They usually hang out with their own kind.