are the enemy units supposed to end up with 6 traits?

You can get bonus by traits training a unit in a leveled fortress.

And by custom designing your units.

Custom design of units maxs out at three traits, doesn't it? Or have I missed an opportunity?

I think it must be on a high difficulty, where the AI gets extra traits.

 

  may i ask, which choice must the player make when a fortress levels to five, to get the extra traits?  I have not seen this.  Does this require a specific blood, or difficulty level?

At level 3 a fortress gets the choice of Impulsive, Charge or Endurance (+1HP per level) to add to units.  It just simply adds that trait to all units produced there (for no extra cost apparently).  There's also traits for the Empire's Warrior's Temple (Bloodthirsty) and a Kingdom building (Emnity) which are both Fortress buildings.

This is probably the bonus traits the AI gets on hard or higher difficulty. If they have traits like strength or bloodthirsty twice then it's this for sure. I find it really annoying.

Bloodthirsty seems way more powerful than emnity.. could be wrong though.

Nope, it is. Enmity is kind of eh, bloodthirsty is quite powerful.

jsut a pesky ai bonus crutch on high difficulty levels; cuz they refuse to let other people write a better ai for them.

Yes, that's the thing missing from Elemental modding capability that is in Civ4: deep access to AI and UI.  Currently modders of Elemental can only tinker, they don't have the AI/UI access that Civ4 has.