1) we can measure the gravitational forces of everything in our solar system. Everything is accounted for.
2) the definition of a planet is well defined. A body has to have a defined orbit to be a planet.
3) Anything large enough to be a planet at the outer reaches of the solar system will likely be a gas giant. How our solar system formed, and how solar systems are formed generally determines this.
4) Bodies like Pluto are not classified as planets but as dwarf planets. It's estimated that as many as 50 such objects exist in the outer reaches of our solar system. We only discovered the dwarf planet Eris in 2005. Eris is larger than Pluto.