I thought they were chapter number 666? Anyway, GK wouldn't be "something unusual," they're used WAY more than they should be, in situations where they don't belong...like in a video game that doesn't have daemons...hmmm. As for Deathwatch, I think they should be handled the same way as the legionaires, as an upgrade to a space marine. Actually, the legion of the damned are a separate and unique chapter, which makes "I'm fielding a blood angels legionaire" a bit silly, while the deathwatch actually allow for that, since deathwatch veterans are eventually returned to their chapters, so....
Do we know for sure there will be no Daemons? There will be Chaos.... and corruption. As far as Legion of the Damned, I wonder how all the die hard fans that are attached to their chapter will feel about a guild mate wondering into the briefing room as a Legion of the Damned character? I dare say not too happy. I had always assumed your elite and hero classes would be tied to your specific Chapter/Clan/Crafworld...