Not the issue. What's important is that planets are the gravitational big boys of a system. For example, other stuff shares an orbit with Earth, but they move in a way subordinate to it. Naturally, when you have an uncontested leader in orbital space, it's going to be relatively tidy because most stuff would have accumulated into the planet or have been ejected from orbit. Ceres and Pluto are unable to bully their neighbors around so they're just dwarf planets.
Yes, but there is no good way to be entirely sure that an exo-planet, regardless of size, has cleared its orbit of x % of its mass (like, the matter subordinate to Earth is a minuscule percentage of its mass, it has the cleanest orbit and Jupiter is number 2). Hate that, and it stuck in the craw of a number of other scientists.