Uhm, Kyuubi wasn't taking control, it's just that his chakra was causing Naruto to go berserk, or at least that was my thoughts on it. I don't really think that Kakashi could have judged if the seal was breaking or not considering he wasn't even sight of Naruto. He was sort of just assuming that because Naruto was using Kyuubi's chakra that something was wrong with it.
Yes, Naruto has a lot of endurance, but that doesn't mean anything when your enemy is making such deadly blows that if you even get hit once you'll likely die. You think that Naruto will be able to survive getting hacked up similar to Kodachi? Possible with the Kyuubi's regenerate powers, but unlikely to live long after Ranma gets more busy. You'll notice that Naruto mostly uses his fists instead of more deadly weapons (Actually, in the Narutoverse, kunai NEVER seemed to get used in close quarters unless someone ELSE has a kunai as well
). Ranma is mostly the same, but still seems to brutalize people to extremes.
Regarding Tsunade, yeah, Ranma would have problems there, but even Tsunade with that seal would have trouble if her head got sliced off. More then likely however, Ranma wouldn't go for that immediately and get wasted. However, on the same tangent, Tsunade probably wouldn't use that seal until too late.
Uh, like I said in previous points, if they use the gates, Ranma is dead, but if he gets them before that, he'll win. Thing with so many Naruto people is that they never use their best abilities until they've tried their other options, Ranma doesn't. You'll notice that Ranma basically kills them quickly and moves on unless he has a grudge against them (in which case he kills them in an even more brutal manner, taking his time).
Actually, Ranma isn't immoral; he's just completely insane. He isn't in tune with reality, so he wouldn't
get that what he is doing is all that bad.
Anyway, it would really depend on the situation. If the Naruto cast attacked him in a group, they'd win. However, if Ranma ran into them one at a time, he'd likely demolish them. Of course, this is all in my opinion.