I've been doing the "sitting in the coop" with my chicken and guineas everyday since i've gotten them, it helps.This is great to hear. I have one Guinea (showed up in my yard out of nowhere when she/he was about 2 weeks old). I put him/her right in with my chicks who were about the same age and she/he was so happy to be with them. Now they are all 10 and 11 weeks old now and I noticed my Guinea chasing one of my pullets and pecking at her. Only did this twice, then stopped. Then noticed a bit of the same feisty-ness the next day. Nothing too severe and he/she stopped almost immediately.
I've received some feedback here on my original post (basically that my Guinea is a he and will only get worse and I should get rid of him). I don't know if I can do that. I'm totally attached to her/him now and all she/he has ever known are chickens as flock mates. For the past few days (since the ''aggressive'' incident), I've made a point to sit in the coop for a half an hour to an hour twice a day - which I do anyway and all the chickens and the Guinea will pile on my outstretched legs - but this time I've been holding the Guinea and petting her/him gently for 5-10 minutes). I haven't seen any chasing of that one particular pullet of late, just general running across the chicken run excitedly with feathers slightly poofed up.
I really don't want to have to re-home my Guinea, and I'm still holding out hope that maybe it is a hen and is just stretching her wings and getting used to being more grown up (but I have no experience with Guineas and trying to find info on their body language is tough). He/she makes the alarm call and I think the single-syllable call, but I've heard females can make both the single and double syllable call, and of course I have no other Guineas to compare her/him to in order to guess whether or not it's male or female.
Your experience gives me some hope, though, that maybe she/he won't turn aggressive and hurt my chickens (and I know at least one of them is a roo and I've heard bad things about male Guineas and roos). He/she normally is pretty mellow and will sit on me or come up to me with the rest of the chickens when they think I have snacks.
Anyway, my dad and I will be expanding their run area to about twice its current size (currently about 100 sq feet, so hopefully will end up being 150-200 sq feet), and I also have a side compartment that is about 50 square feet, so maybe if it is a he and he turns aggressive, I can keep him in there (that way he'll be separate from the chickens but still part of the flock (because he'll be able to see them all through the wire)).
If you have any more advice you can offer, I'd really appreciate it. I've been feeling pretty down about the whole thing since I'm afraid I will have to re-home her/him after all and I'm afraid that might make him/her depressed since she/he has only known chickens as family members.