I have read the infernal and there is soo much i love about it, but i think i am going to change the nature of the abyss vs Hell. I am talking about the first Angel war which was angels fighting against the endless demons from the abyss. That the abyss was the dark to God's light and sought to destroy or curropt creation. The Angels fought against the endless hordes. When Lucifer and his cronies rebelled against God for not bowing to man, The fallen angels became devils and were thrown into hell which was a gate to the abyss. So they hate demons even more than humans and fight to keep the horde back, punish the wicked, and lead humans into temptation.
Devils can be strong or weak but they are more lawful and follow a perscribed heirarch, where as demons are chaotic and only the strong rise to control demons.
How do people feel about this slight change>
Enjoy. 
Yeah, have fun with it. It'll never make ArM canon because it's not what people believed in mediaeval Europe (at all, as far as I know), but don't let that bother you.
You might want to think about whether you want to split Infernal auras (and Might) into "Diabolic" and "Abyssal" auras, with a slightly expanded realm interaction table, but that's not going to be hard.
I don't think there's any such distinction between demons and devils in Ars Magica, or any such conflict between them. The term "devil" seems to be reserved pretty much for The Devil whereas other Infernal entities are either celestial demons, or fallen angels that fought on The Devil's side against God, or terrestrial demons which are either Infernal ghosts or the progeny of other demons. I doubt any of them get on especially well but I believe the demons vs devils idea comes from D&D, as does the Abyss and Hell being seperate places.
Do what you like. Some Criamon believe demons are what's left of humanity at the end of the process of rising Chaos.
For some more explorations of the ideas of Angels fighting demons from the abyss, Stephen Brust's "Reign in Hell" may interest you.
That is not entirely true. The first Angel war, was mentioned in a couple of ancient texts from civilizations other than Hebrew, Greek, or Roman. This was during creation when God created angels to fight the hordes. The question is how does this help. According to Dante's infernal the 3 ring of hell was the abyss that was endless (at least I think it was the third ring) But he doesn't expand on it much. The abyss is one of the levels. By combining myths from different ancient cultures that believe in angels and devils we find this whole war exists. Furthermore to make more sense and relate it to Hebrew/Greek/ Roman mythology, why did God create warriors if there was nothing to fight? All myths agree that Angels were warriors of Gor and charged with protecting his creation since before man was created. If man was the cause of demons and the devil then why make warriors before man gacked things up? DnD capitalized on this ancient myth of hell having seven layers and the abyss having unlimitied levels, and seperating demons from devils but this was drawn from mythology. And since a few philosophers of that era believed that the abyss I need to reread it. And even in the bible it refers to the abyss as seperate. I just don't think this whol section of fallen angels which by their very nature are finite and demons are the same. I think that in a world full of real life demons vus angels in looks etc that people would realize it is different. Of course, LOL, it is my game and I can do with it as I like. But angels were warriors before man, who did the fight? Why were their created? if it took man to bring in evil?
great book i have read it.
Well, I think the conventional belief was that there was no evil and no evil hordes until Satan and his angel followers rebelled against God. However the fact that there are all sorts of contradictions with what people thought about the Devil, demons and Hell that it's wholly possible that what's really going on is not what anyone thinks at all. That also means it can be whatever you want it to be! 
Which myths are those 'all myths' you are refering to? Because while I don't claim to know all myths, those I know don't talk of angels as warriors of god prior to lucifer's rebellion as far as I understand. Agents of god, sure...