Mix Matching Chaos Armies

If this is in the wrong rules area feel free to move it. At my LGS I overheard a rule for chaos space marines that said that it is possible to use some stuff from the chaos daemons codex. Like say I want a Soul Grinder in my heavy slot as a CSM instead of what is in the codex. Is this a legit rule found in a white dwarf or no? Thanks all.

Only with allies. Can ally in a soul grinder (to use your example) as long as you take an HQ and a troop from the Daemons book. But you can't say "I'm taking a chaos lord, two squads of cultists, and a soul grinder". Same way as you can't take a Codex: Space Marines list and toss in a Baal Predator from Codex: Blood Angels into your fast attack slot.

What you might have overheard is the one of the "rumors" for future chaos supplements which would include 4 books, each dedicated to a different chaos god and will supposedly allow you to take units dedicated to that god from both chaos marines and daemons without using the ally rules. But again, still rumors for the time being.

^_^ Thanks gang! Well I can use a defiler still so it is all good.