Is there a risk of the two cats not getting along and having to be kept separated like extra-small pitbulls?
Yes, but usually this is a case of either cat having had bad experiences.
We had a 9 year old cat, who had lived under the rule of thumb of a very dominant male before we adopted her. She was beaten and generally forced into submission.
We soon added a kitten to the household, and the old one hated her. She turned violent, and after she realized that I'd intervene on the kitten's behalf, she resorted to more passive aggressive actions like claiming the litter box, or sitting by the food because the little one was too afraid of going near her.
Don't let that scare you off, it's not all that common. Most cats are known for their generally indifferent attitude, and this extends to their social interactions as well.
We now have two sister cats, and added a kitten into the mix. Here, you can see the difference:
- One sister is very passive. She likes sleeping and loafing around. The little one tried to play with her. She would go and lie down somewhere else, but after he pestered her too many times, she would chase him out of the room. Result: These two mostly ignore each other now.
- The other sister is more playful, and used to try and get her sister to play (unsuccessfully). She actually plays with him more often. But he has more energy than her, and she wants to loaf around at times too. Result: These cats interact much more. When he gets too playful or she isn't up for playing, she will hiss at him a few times until he gets the hint. But they don't ignore each other.
Unless you're getting two unfixed males or cats with emotional baggage, the odds of them violently disliking each other is minute. Even if they do, they will generally resort to ignoring each other instead.
If you're trying to put two males together, only put them together after they both been neutered for some time (so the hormones have left their system).