This is HERESY!

Getting serious, I need to ask some stuff.
Wouldn't this fit better in nations roleplay? 
How big of a part would good old chaos play a part in this?
Will those planets be apart of the same sub-sector? Will they be close to each other?
What part of the Imperium will this take place? Seeing as the eastern fringes of the empire is being assulted by the Tau and the Tyranids.