We rescued a guinea in much the same circumstances last year in summer. She lost her entire flock which was guinea and chickens. She spent all her time looking at herself in the storm door on her owners' porch. She was lonely and not really being looked after. They were in a messy divorce and no one was home a lot of the time. We took her (we were told it was a he BUT she Buckwheated the first day here. She was about a year old I think.
I had her penned alone for a few weeks, in sight of the chickens after the first. I was going to have her confined for longer but she escaped while my son was feeding her. She immediately joined the chickens. Initially she slept with one flock and ranged with my other. She never left the chickens and has never hurt any of them. The chickens mostly ignore her. She did entertain us endlessly because she did not allow the two roosters to fight. If they started sparring she would get in between and separate them. It was hilarious since her feathers kind of look like referee uniform stripes!! And seeing our big tough rooster dragged around the yard by his hackle feathers by this little bird half his size....who wouldn't laugh at that!!
This spring we got Guinea some new guineas. There are nine of them and she will not have anything to do with them other than protecting "her" chickens from these naughty youngsters. We wonder if that will change next spring when she wants a mate. She spent all the spring just gone plaintatively "buckwheating" all over the property but she always returned to her "flock"
So yes it can be very successful. I think she was just so desperate for a flock that the chickens have kept her here.