Im worried that as Leo is so territorial he just won't accept another one around.
I think it makes it harder that he is an indoor cat too.. are yours?
The last thing I would want is to have to re-home the newbie as Leo hates him so much.. It would break my heart. I suppose its always a risk whenever anyone introduces a new cat to the existing one/s.
From what Ive read about Maine Coons, they are very laid back and easy going. Is this true of Leo? If so, then I dont see that he will have any problems with another cat in the house. If you are worried, then you can make the introduction gradually, ie. keep them in separate rooms and gradually allow them to sniff eachother.
One thing with Reflex was that some times I used to see him, he used to look very glum and bored. He would sit in his bed with his head hanging over the edge, looking at me. He came from a breeder with a big cat family and I knew he wanted feline company. So I knew that dispite any early fighting, he would definitely appreciate another cat in the house.
When I introduced my 2, I didnt do it gradually, as I knew they were both young and even they fought, they wouldnt be able to damage each other as they just didnt have the power to do so. Also, as they were both similar ages/sizes, this helped, where one cat wouldnt get an overwhelming advantage over the other, during the introduction.
Reflex was getting aggressive at times and I could see Mr Bengal couldnt handle him. During these times, I would place Reflex in a cat carrier for 5 minutes, to allow him to cool off. Of course, a day later, Reflex was fine to share his territory with Mr Bengal.
My problem with Leo would be that he is a big cat and is probably pretty strong, so if a kitten were introduced, he could do damage, intentionally or unintentionally (he probably doesnt realise how strong he is).
Providing Leo isnt overly aggressive and is easy going, I dont see any problem introducing another young cat to the house. I think many stories you read on the forum are people getting overly worried.
As an example, the breeder I bought Mr Bengal from had cats of all ages and 3 different breeds. In particular, Mr Bengal's father was incredibly fit and strong. The fittest cat Ive ever seen. He was like a bodybuilder, mainly due to the fact that he had a HUGE cat wheel that he used to exercise on constantly. And even he was able to live with other male, un-neutered cats. They acknowledged that at times there could be territorial disputes, but these would sort themselves out within days.
Obviously some cats are totally mental. Reflex's father was like that and couldnt live with another cat in his cage. But thats the Egyptian Mau breed, which is totally different to Maine Coon.
Take what you read on a forum, with a pinch of salt. What does your gut tell you - can Leo handle another cat in the house or not? And if so, what age/breed would this other cat be?
As an example, Reflex is very aggressive and is very strong now. He struggles with me a lot when I hold him and is a lot stronger than the Bengal (who is bigger). If I brought in another cat, I would ensure that it was a physically strong breed - probably a Savannah - as these are taller/larger/stronger than Egyptian Maus, so eventually, Reflex would be unable to exert his dominance over it. I would also ensure that he was the most dominant of the litter - the runt might be too submissive and might get beaten up by Reflex. Mr. Bengal is fine with other cats, so I'd have no worries from his side.