Doc wrote:
Would demons fight other demons for dominance of a town or would they share ? Say there are two cults trying to take over a town, is there only so much "demon food" to go around, would one of them have to move on to someplace else ?

I think the demons always work together as a nameless force. But really, how could this be relevant? The demons indicate how far a town has slid on the scale from 'obeying the King of Life' to 'living a life of false doctrine and sin', and it is as such, indicators of obedience and disobedience, that they play a role in the game. They are not themselves the focus of play, and warring cults, the ecology of demons or the psychology of demons are all very much besides the point, in my opinion.

The game as written offers support for a range of treatments of the supernatural, from just plain bad luck, through a kind of karmic retribution, to full-tilt supernatural horror.  It's up to each group to decide how to treat these elements.  Just leafing through my GenCon edition, I can find support for a variety of approaches.  For example, while there's nothing stopping Victor from declaring that demons are a nameless force, there is, on the other hand, this, in GenCon Ed. p. 23:

To Drive Demons out of a House: Make the sign of the tree...If you know the demons' names, use them!

But I think Victor is right when he suggests that abstract cosmologies are rather beside the point in DitV.  The game focuses on the concrete, not on an abstract set of rules.  Treatment of the supernatural will take shape as you play, determined by the GM's playing of cults and possessed people, by his or her set-up of how the demons attack, and by the players' understandings of an emerging continuity.  A kind of coherent demonology or cosmology will emerge from cumulative treatments.  As I understand it, building a demonology in DitV is more common-law than statute: it's built up, precedent by precedent, without an overarching superstructure outside the rules themselves.  Or to put it another way, while other games might dictate that the behavior of demons is based on a theory of demonology, DitV reverses that relationship and grows a demonology out of specific, concrete Instances of play.

Having said that, I could imagine multiple cults contesting one another for control of a town.  But there's no way, really, of knowing whether those cults are truly commanding different demons; it could be that the enmities between rival cults are feeding the same demon, or it could be the demons are using the cults as catspaws, or...  You get the picture.  The way I look at the demons is that they are a force against the Faith; however that takes shape, that's what they are.  So for my game, I would never pit two demons against one another, though I just recently had one cult which deployed another cult as cannon fodder against the Dogs.

In response to your second question, some of the other threads on this forum demonstrate that DitV is highly portable.  I may be wrong, but it seems all you really need are an episodic structure and characters with the authority to stand in judgment.  For the purposes of Fallout, sure, I think you could transport DitV into that setting pretty easily.  You wouldn't even need to make many changes.  You could use the hierarchy as Vincent has written it.  You could imagine setting-specific Sins, such as "Using unclean technologies" or "Giving rise to mutation" or something.  The Doctrine that emerges might be frothing evangelicalism laced with racism, or it might sound like something out of a 1950s Civil Defense manual.  Maybe the Dogs want to preserve the purity of human blood.  Maybe they want to avert the renaissance of unclean technologies.  Maybe they are actively trying to promote the rebirth of science and learning.  The potential is certainly there.