Colubrid ??

colubrids are a very wide range of snakes, so do all rear-fanged snakes fall under this category or how do they decide what is a colubrid and what isnt??

All rear fanged snakes are colubrids, but some share more in common with elapids, etc. An example are the beaked snakes - which may be more closely related to elapids than other colubrids.

Colubridae has customarily been the taxonomic dumping ground for snakes that didn't neat fit into other taxa - that's why alot of people refer to colubridae as the "dustbin".