I was looking at this earlier today. It says all units in the detachment must have R&H or CSM faction, but does not have a rule that overrides the BRB rule of all units in a detachment must have the same faction.
actually... both should be allowed in the same detachment now that I look at it closer
Under Restrictions of every other detachment I've randomly spot checked it says all units have to be part of X faction, and only listing one. The BRB does not state that all detachments have to be all the same faction, just that the ones mentioned (CAD and AD) have to
If you wonder why I'm picky about nobody messing with the CSM file, this is why. Right now it's a clusterfuck that is set up how it is because that's the only way it can all be in one file in the current version of BS.