Yes, they're fictional, but it's a good start, since after all it proves that modern-day researchers weren't the first people to think of classifying planets by their habitability characteristics. Also, the simple "class [letter]" scheme is easy to remember and use; I sure hope they don't come up with some arcane, complicated system instead. Finally, they should definitely use "Class M" to refer to Earth-like planets simply to pay homage to Star Trek. Everyone and his brother knows what a "Class M" planet is, as long as they watched some Star Trek within the last 50 years.
It looks like you got your classes from Star Trek too, as seen here, but with some differences. I'm not sure where you got Class Q or I. The system probably does need a little revision though. Class H's "generally uninhabitable" doesn't tell you why. The Class P (see appendices) for icy planets is a good example. Class N for "sulphuric" really isn't sufficient; Venus is more like the Class Y "demon planet" except there's no dilithium-based biomimetic lifeforms, but the fact that Venus is so hot is important it needs to be classified that way. If a planet is too cold or too hot to live on, that's an important factor for humans. Same if there's no atmosphere. A planet (or moon) that's not too warm or hot but has no atmosphere can still be inhabited using domes or other sealed habitats, so that should be a class by itself. Mercury probably wouldn't fit there however, because it's much too hot. But it's hot in a different way than Venus, so they should have different classifications (hot because it's too close to the star, vs. hot because it has a thick atmosphere and runaway greenhouse effect). Finally, moons and planets should be classified together. The orbital path doesn't really matter (except insofar as it affects the climate/temperature). There could very well be Earth-like moons out there somewhere, so those should be Class M (like the moon in "Avatar").
So here's my proposal which borrows from ST:
Class M - Earth-like, small, rocky, oxygenated atmosphere, right temperature
Class D - small, rocky, little to no atmosphere, right temperature, inhabitable with sealed habitats (e.g. Mars)
Class J - gas giant (any size; this may be expanded later after we explore more star systems and decide we need to classify them further)
Class E - small, rocky, little to no atmosphere, too cold (e.g. Pluto)
Class F - small, rocky, little to no atmosphere, too hot (e.g. Mercury)
Class G - small, thick atmosphere, too hot (e.g. Venus)
Class A - very very small, not spherical (e.g. moons of Mars, captured asteroids)
Class B - very small, spherical but extremely low gravity (e.g. Sedna, Ceres, Pluto, dwarf planets in general)
I'm probably missing something here, perhaps planets with only liquid surfaces. I avoided calling Venus "Class N" because it sounds too much like "Class M".