Allergies can cause cats to drool excessively, also. My cat is allergic to conofite lotion. We found this out when our vet prescribed it for him, but when we started using it Dokota got lethargic, disoriented, and drooled like the dog in Turner and Hooch (a Dogue de Bordeaux). So there may be something that the cat is allergic to in the home (food, medication, household cleaners, etc.). I would definitely seek the advise of a veterinarian and discuss every possibility.
As for the cat and others in the house not settling in, it may take a while for them to get used to each other. Sometimes they never really "get along", they may just tolerate each other. I know people who have multiple cats that just keep their distance from each other. If kitty A walks into a room where kitty B is then kitty B will simply walk out. As long as they aren't fighting I think they are probably ok.
"The fact that man knows right from wrong proves his intellectual superiority to the other creatures; but the fact that he can do wrong proves his moral inferiority to any creatures that cannot." ~Mark Twain, What Is Man, 1906