I'm fine with the idea of mixed regiments. Although it's a bit of a lore stretch, I can get why you would want to play something like this (more diversity between characters)

So yeah, I would really love to play this game, although litterally being a copy of the Deathwatch would probably be a bit boring. Maybe have the unit be a part of a commander/commissar's special forces unit or something like that? (Please no Inquisition T.T)

And finally I would agree with [@Whoami] that the Tanith shouldnt be allowed because of how few they are in number. I would also like to add the Death Korp to this list as they are not really known for being sociable and thus would only be interesting in combat scenarios, but not in the RP itself. But hey, that's just my opinion.