Just a quick question to anyone who knows more than me about this. I recall reading somewhere (but naturally I forget where, which would make this so much easier) that between the chaos gods, Tzeentch and Khorne have particular emnity towards each other. I have no idea if this is cannonicly true or not, I just assumed they all kinda didn't like each other but don't generally fight amongst each other too much. How about Nurgle and Slaanesh? Are any two of the gods directly enemies of the others? The Lexicanum was not as helpful as I had hoped.

This has largely been backed away from in the fluff, but you're correct in that they're rivals. Khorne and Slaanesh are enemies, and Nurgle and Tzeentch as well. This all started back when GW released a couple of supplements for Chaos in both fantasy and 40K, called Realm of Chaos. One was subtitled Slaves to Darkness and covered both Khorne and Slaanesh, and the other was Lost and the Damned with Nurgle and Tzeentch. These books, while dated in terms of artwork and retconned like crazy, are an ABSOLUTE TRESURE TROVE of awesome chaos stuff. They are worth picking up and reading, and they are much more unabashed about how messed up the Chaos Gods should be.

IIRC Khorne and Slaanesh are opposed because they represent roughly opposite ends of the Chaos cardinal points and their philosophies are so different. Khorne being the patron of unthinking anger and wanton slaughter, and Slaanesh hedonistic delight and sensual pleasure (not just sexual, but of the senses). A similar idea exists between Nurgle, mirthful and doting over his diseases like a twisted father, working towards the decay of all things (seeking a sort of uniformity) and Tzeentch carefully scheming and twisting the threads of fate to unfathomable goals.

The idea of Khorne and Tzeentch being enemies stems from the idea that Khorne hates sorcery and magic, while Tzeentch is the unabashed master of it. You are right in the sense that they all fight and bicker, but their actual fluff-based enmity is as described above. I think this opens up all sorts of awesome ideas for DH if you run a Malleus campaign!