We have 12 cats, 4 generations of one family, 2 brothers and 4 completely unrelated cats (6 girls, 6 boys, moggies and purebreeds). We have not had any issues integrating new members (following the separation rule to begin with) providing you give them time and space and don't just throw them together on day 1.
With that being said I cannot rave enough about our Maine Coon!!! He is so laid back, so easy going, he is not nervous nor gives off any vibe at all that made any of cats see him as a threat, and after 24 hours of bringing him home he decided he was ready to mix with 10 others (he is number 11). We also have a daughter with neurological 'problems' and he is so aware of her needs, so calming, so peaceful, he has become her 'therapy' cat. He is not really a lap cat for anyone else, but he waits at her door every morning to go wake her up, but only after climbing in bed with her and having their cuddle, on a bad day he won't leave her side. He is incredibly patient with her lets carry him around, brush him, snuggles with her.
If you are looking for a therapy cat in as much as one can be one, please! read up on Maine Coons, our Nova is a stereotypical Maine coon in every way and our daughter has benefited so much by having him (he wasn't intended as a therapy cat but it just happened). Pic because, well, he is also incredibly beautiful.