The actual planet itself is very active geologically and has a whole system of moons. The only reason you'd say it's very unlike the other 8 is due to orbital dynamics, i.e. due to other bodies besides itself. This is a strongly anti-planetary-science bias, IMHO.
Sooo... mass and composition are irrelevant? Because Pluto (and its fellows, like Eris) are smaller than a half-dozen moons. Pluto has less than _five percent_ of the mass of the smallest planet, Mercury. It is also mostly _ice_ unlike the inner (rocky) planets and the gas giants.
And variety of solar-system bodies are geologically active, from the inner planets, through numerous moons, down to comets and asteroids, and probably many other Trans-Neptunian Objects like Pluto (many of which also have moons). Your proposed criteria fail to be useful.
