
Mysterious Stranger |

No fear conditions do not stack there are specific rules for this. Essentially they are more advanced forms of the same condition. In most cases they actually increase to the next level. So a shaken character who gains another shaken condition becomes frightened. If you are frightened and gain another fear condition you become panicked. The demoralize by the intimidate skill an exception to this and is stated in the description of the skill.

Saldiven |
Conditions are often described as being a type.
Shaken, frightened, panicked and cowering are all fear effects. The modifiers don't stack; you just take the most extreme of them, for example.
However, you can have someone who is Sickened, Shaken, Pinned, Dazed, Dazzled, Blinded, Bleeding, and Flat-Footed all at the same time, theoretically.