The temperament of chameleons is generally fearful or aggressive towards the keeper but completely hostile towards any other living creature. Even the resemblance such as a mirror or picture could put a chameleon on edge. Chameleons are generally solitary by nature and rarely cross paths unless mating. The exceptions are the pygmy chameleons. This means that chameleons may not be housed together (again with the exception being pygmy chameleons)or with other animals. Regardless of whether the other inhabitants are nocturnal or not it always ends badly.

Veiled chameleons are famous for their nasty disposition. However, all are not necessarily nasty. It is true that usually the males are the nasty ones. I firmly believe each chameleon possesses its own personality and until you actually own your chameleon you won't know whether it will be aggressive or not. It male be friendly and tolerable for now but once it reaches sexual maturity, you can say good bye to your cute friend. 

 

When keepers mention one type being generally aggressive, it is a generality in which most of that species are that way, but not necessarily all.