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Its true that most chapters don't revere the Emperor as a god, but many do, and these radical chapters would conscider the othes to be heretical. I know of some flare ups between chapters like this. The same goes for non-codex chapters and codex chapters.

They are only chaos if they worship the chaos gods. Sure, pirate armies are made with the chaos list, but there are chaos pirates, and normal pirates.

Its possible. Many people would say, no its not, because to Marines are nothing but death machines. But if they were corrupted and became mercs then I can see it happening.

Make them follow the Greater Good. Makes enough since: thye are heretics, they help others, and they don't believe in the Emperor as a god.

But as long as you can justify it as for the Greater Good, then it works. If the marines raid a chaos weapons depot, its for the greater good. If they kill an imperial lord a day before he launches an assault on an innocent system, its for the greater good. you get the idea?

You could also use simply corrupted morals or use of unnecessary force.

just think about it.

Argh... Could you please stop quoting each other? There's no way someone would like to read again and again what's been written before and it uses up a lot of space. An interesting discussion can be ignored by many people because of this.

Such a chapter wouldn't survive long - it would be denied any help from the Imperium and what's more important for SMs, Adeptus Mechanicus. They wouldn't survive long, just like Relictors, who weren't necesarrily bad, but since they supported the radical fraction of the Inquisition it didn't took long before they were hunted down.
Each chapter has to send some of it's gene-seed to Adeptus Mechanicus for signs of any mutations and are often checked by the Inquisition for any abnormal behaviour as well. I believe that 13th Company didn't make it for long as well, since they were not accepted by the Imperium. And they had no way of making new brethren.
There's always Cursed Founding. You can simply make a Cursed Founding chapter that fallows it's own unknown rules and is both enemy and friend to the Imperium. Considered enemy and trying to be a friend. Or create a custom (in)famous chapter that is sure to fall but before that it can do some things. It won't probably last that long because some other Space Marines chapters would hunt it, being a disgrace in the eyes of the Emperor. But for now, they're alive.
The greater good wouldn't really work for Space Marines... Serving the greater good is an instinct for Tau, just like serving the Imperium is instictive for Space Marines. While it is possible for a simple guardsman to belive in Tau ideals, Space Marines are too indoctrined by their chaplains to belive in such a fable as 'Greater Good'.

And I try not to read the quotes but it takes some time to scrool everything down and search for valuable info. Not using quotes all the time is quite... neat.

Back than the Imperial Creed did not exist as the Emperor belived that every legion and Primarch knew their alliegance to him. Only after the Warmaster succumbed to chaos and betrayed was the need for indoctrination seen - to ensure that something like this would never happen again. The Imperial Creed is taught by each and every chaplain (although I'm not sure how it is with things such as Rune Priests or Iron Priests).

And there is one general stereotype - Index Astrates. Some chapters differ less or more from the 'standard' but they generally follow similiar rules. Otherwise they're not considered part of the Imperium and that means only death. There are some exceptions like the Space Wolves but they're mostly the first founding chapters, or like SW, a legion, and are famous all over the Imperium.