You can add as many characters as you want (just make sure their names match your factions list), but I haven't even looked at how it assigns portraits.
Triggers were pretty simple when I added all 9 Roman temples and corresponding priests (for retinues) to the Julii faction. WhenToTest was after the shrine got upgraded to a Temple, except in the case of Neptune, which "tests" only when you build a new Admiral (so that land units won't get the bonus meant for naval units). To test the whole mess, I made the chance of getting a priest 100%, and then built a temple for each god; sure as hell, every general in the city at the time got the priest. So if I stack 5 generals in Capua and upgrade my shrine to a temple, all 5 get the priest.
So it means the percentage chance. Using the example you posted, if a general withdraws from a battle, he has 100% chance of getting "Indecisive Attacker", and so on. As I did with the priests, you can adjust this number to increase/decrease probability.
Bookmarks