Traits from NPC's can exist with creature or culture traits, as well as each other. But, the three creature traits can't exist together. Pyromaniac and Immune to Fire conflict with each other. The chance to get the latter is higher than of the former. But, sometimes the game can choose only one hidden trait at birth if more than one are possible. If one of those traits is a creature trait, then the NPC trait won't be inherited. Although, it is proven that a NPC hidden trait and a creature trait can exist together during a Sim's lifetime. An obvious example is Beau Merrick. Sims can have more than one hidden trait, if that's the question.