Thanks for the pointers, it's why I came here. I would like to ask a few questions then and explain my view on some points and would love more feedback since you sound pretty knowledgeable.
First off, would the story work better maybe as a two-part or even trilogy of shorts, that way I can plot out the pacing a bit better?
Hello, WTAnderton, welcome to the forums! I was hesitant to add to this discussion before, because my grasp on 40k lore has slowly been decaying over the years lol. Darkreever's lore is probably more sound than mine, but I'll try to help in any way that I can
As for splitting your story into a number shorts, it really comes down to whatever you feel best allows you to tell the story. If making your short story into a trilogy will better enable you to progress character development and plot, then go for it
Instead of the Inquisition, maybe Faris could be drafted into the PDF by the plantery governor? Does the planetary governor actually have that much power? If so, then perhaps he could also have just requested aid from a nearby Space Marine chapter and I won't have to get into the Inquisition or Deathwatch too much if at all.
The Planetary Governor, I would imagine, has ruling authority over all Planetary Defense Forces unless a higher authority (such as an Inquisitor) steps in and takes over for him. So I think that plan could work, also I believe he would have the authority to send a distress signal to any nearby space marine allies as well. I don't know who else would have that authority, plus it allows you to sidestep the whole Inquisition/Deathwatch problem.
I do like doing xenos rather than daemons or general Chaos, though with the overarching plot I had in mind beyond Faris' story, the xenos that would make the most sense would be eldar or necrons. On the surface is there really any problem using those? Like if the planet was a necron tomb-world, would it be impossible to be an agri-world?
Actually, I'm not sure, I would imagine a tomb world would have once been a barren world. But being dormant for ten thousand years, I suppose life could be brought back to them. Perhaps your agri-world was a planet extensively terraformed by the eldar after the War In Heaven, before it was conquered by the Imperium?
As for the Space Marine chat, any problem with that?
I don't think this is a problem, depending on the Space Marine's personality and circumstances of their meeting.