Editing Warhammer 40,000/Tactics/Kill Team(8E) Guide and FAQ (section)

==Can I soup [faction] and [other faction] together?==
No, every model on your roster must share a FACTION keyword.

'''Daemons and CSM both have the Chaos keyword'''

That’s not a faction keyword. Faction keywords and regular keywords are both keywords, you can use them for abilities, tactics, etc. but only faction keywords are used for determining which models you can put on your roster and which tactics you get access to.

'''What if I’m not battle-forged? The book says that’s optional.'''

It’s not really. Matched play requires battleforged teams and rosters with only one faction keyword. Narrative missions require battleforged teams. Technically, you can mix factions on your roster for non-campaign narrative play (that restriction slipped through the cracks) but you can’t mix them in a team anyway. 

You can do what you like in open play but bear in mind that it’s only being battle-forged that requires you to obey the points, unit and specialist limits. So, for example, 100 terminator gunners with assault cannons who are all heavy specialists is perfectly fine. You also don't get command points, making some narrative missions unwinnable. Pretty much everyone uses battleforged kill teams. If you had a play group with other restrictions you wouldn’t be asking us.