But presumably outside that mass range, gravity would be too great or too low to be class M anyway, proceeding on the assumption that a smaller planet's magnetic field would be too weak within the habitable zone to sustain its atmosphere long enough for life to develop. Although having said that, I think Venus' magnetic field is not all that strong presumably its atmosphere is so thick because of all the volcanic activity?
I suppose a smaller planet would be able to maintain an atmosphere if it has a regular enough supply of icy meteorites to restock it but its hard to see that that could be maintained long enough for complex life to develop.
There are probably only so many classifications that occur in nature so other classifications would need to be for unusual planets that require further investigation. If the demon planet is classified as such because its atmosphere and radiation make it dangerous to both ships and personnel even with protection then those should be considerations taken into account in the classification system.