As a setting, 40k in particular is extremely open, with lots of "gaps" in the basic material to fill in your own ideas. For example, there are supposed to be about 1.000 Space Marine Chapters, yet only a small fraction of those have ever been named anywhere, leaving the rest for gamers to make their own. The different livery and heraldry as well as Chapter culture likewise exist in part to make the various armies more "personal", but also to enable players to insert their own ideas into the world. Example: you like samurai? I'm sure there's a Space Marine Chapter for you. It may not be mentioned anywhere yet, but the galaxy is a big place.
And even beyond these gaps left by the official material, the IP allows for considerable artistic license, with many of the official and officially licensed sources contradicting each other, which has in the past been explained as the books that we as real people read being distorted by legend, myth and propaganda, leaving it to decide for yourself how your Wh40k looks like.
The Deathwatch RPG in particular clashes with a lot of GW's original background, yet both visions of the setting are equally valid. In the end, regardless of whether you make up something completely new, or rather change something that already exists (like Space Wolf history, organisation or Chapter culture), what matters is that you and your group like it (as your changes may affect the game for everyone else), so I'd greatly recommend talking to your GM and fellow players to make sure you're all on the same page and play on common ground.
As much as the many different official sources deviate from one another: within your little group, consistency is a must.
Good luck with those creative juices!
Edited by Lynata, 05 February 2014 - 01:11 AM.