OK, I've figured out PS and PPS (modding the spawning of heroes) as well:
In the *insert campaign name here*.xml file you'll find the file the campaign uses to spawn heroes (e.g. "Story_Plots_Empire_AI_Heroes_EXP"). This file references to another file (e.g. "Story_Empire_AI_heroes_EXP.xml"); you can edit both the reference and the referenced file (in my case, I modded the scenario to use "Story_Plots_Empire_AI_heroes__15BBY.xml" and modded this file to reference to "Story_Empire_AI_heroes_15BBY.xml").
In the referenced file (e.g. "Story_Empire_AI_heroes_EXP.xml") you can change spawning location and spawning condition of heroes as well as adding new ones (e.g. I've added Kyle by adding the lines
<!-- Spawn Starting Heroes -->
<Event Name="Spawn Kyle">