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You can get things like the Trojan asteroids that hang out at Lagrange points ahead of an behind a planet, but most of the time if they find similarly sized planets close to each other, they end up being binary planets or Roche worlds.

What they mean by 'clear their orbit' isn't that nothing ever crosses it, it's that no other similar objects share a similar orbital path in the same general area. Pluto has 'neighbors' who are similarly sized that are rather close by in astronomical terms, enough it is not in a truly 'cleared'...

Pluto is the King of the Dwarf Planets now because it has not cleared it's own orbit the way teh 8 main planets have, even if it fulfills all other planetary criteria. The other option was making Ceres, Eris, and a few other recently found ice dwarves/TNO's into full planets, which is kinda...

The likelihood of Pluto being a captured exo-planet is low, and there are others like it in similar orbits out past Neptune. None of the main planets, and very few of the drawf planets, have any real possibility of being from outside the Oort Cloud, much less a rogue planet that was captured...