The four rocky planets -- Mercury, Venus, Earth, and Mars -- obviously formed differently from the gas giants -- Jupiter, Saturn, Neptune, and Uranus -- hence, there is no astrophysical reasoning behind it, and as you said, more will be discovered, so why the rush? The ONLY reason for the reclassification was the blind panic over US planetary discoveries. Mike Brown, who agrees that Pluto "had to die", but who was thus denied discovery of a tenth planet, also saw the creep from Spain get off without censure or any other symbolic punishment for hacking and stealing Brown's research. Obviously the hacking and stealing was a real issue that should have been dealt with, and was ignored instead.
Actually the inner rocky planets formed similarly to the outer gas ones except that the inner ones were close enough to the sun for most of their lighter material—hydrogen and helium—to be forced outward during the sun’s T-Tauri phase of high solar winds. Otherwise, they all formed as small planetesimals coalescing into larger ones until there was no more raw material in their general orbits. There are regions of instability, however, where this process remained incomplete. Namely the asteroid belt and the later discovered Kuiper belt, of which Pluto is part.
Personally I’m not particularly worried about what “defines” a planet. Naming conventions are by nature often ambiguous to a degree, resulting in exceptions to the rule. There is no real difference between a mountain and a hill, for example, except some arbitrary height from base to summit. Originally, the term “planet” referred to a star that wanders instead of remains fixed. So under that definition, the earth isn’t even a planet.