Developing Leaders is what allows you to select traits ingame. Without it, you're limited to the leader presets for the whole game and most combinations on Complex Traits are not reflected because Complex Traits adds a lot of new traits into the mix and leader defaults haven't been updated to consider them - if they did it would take a special leader set or playing without complex traits would cause a crash when a leader has a trait that doesn't exist in the game. If you want the original starting leader traits, you can avoid selecting Start Without Positive Traits and you'll start with the leader defaults then go beyond that with selections later, BUT, again, those defaults are coming from a very limited pool of core game traits without Complex being in play.
There are some traits on Developing Leaders, where the negative trait is completely opposed to the positive trait line, which makes it in essence the same thing to take a trait and then remove it. There are a few that aren't compatible with each other in the religious traits section even if they aren't direct polar opposites. Charismatic and Strategic should be possible to combine but I'm not 100% sure on Impulsive and how that might interact with either one without looking more directly at the Complex Traits thread.