Hi,
First post on BYC so I'll be brief.
We have 45+ Guineas in with 40 RIR chickens and they get along without any problems.
When you first get your Guineas, either babies or adults, you want to keep them in the coop for at least 6 weeks so they learn that the coop is their sleeping place. Allowing them to roost in trees is begging for problems as they can't see in the dark and Owls or Coons will have an easy time getting them.
They should be fed Turkey Starter or Game Bird Starter as they do require a feed that's a higher percent (18) then regular chicken feed (12-14).
Most Feed Stores sell them starting in May. As stated by other Poster's, babies (Keets) must be kept in a warm (95), draft free, location for the first week and then drop the temp by 5 degrees each week until they're 4 weeks old and then they can take the normal summer temps. They can't be allowed to free range until they're at least 14 weeks old.
Hope this helps.
Bob