So all of the promotions are cumulative, and not replacements?
Meaning Guerilla 1 is +20% and Guerilla 2 is an additional 30%?
Yes, in fact most bonuses are cumulative.
As an example lets take an Archer, it can get both Geurilla promos, starts with a natural +25% (extra) bonus on hills and +50% (extra) in cities, can fortify and have city defense bonuses.
So for an archer with Guerilla (G) 1 and 2 fortified in a city on a hill with 40% culture defense will get
25% (Fortify) + 50% (G 1+2) + 40% (Culture defense) + 25% (Hill) + 50% (Archer city defense bonus) + 25% (Archer hill defense bonus)
For a grand total of 215%.
In defense there is one exception, with walls and castles. The defense value of the tile is the highest of your culture against you walls and/or castle
i.e. if your culture defense is 40% and you have a wall for +50%, the total defense will only be the 50% of the wall,
and if your culture was 60% and you had the walls for +50%, the total defense will only be the 60% from culture
