I've had them as starting playable faction and can view the family tree - albeit it was in a rather mangled RTW mod I was running under BI, with Slavs internal name added in (the one I was using when testing the stuff about emergence) - you're right about that other link to them controlling emergence for the campaign, but they don't have to be the emergent faction and can start normally, but presence in sm_factions does allow other factions to use emergence.

I'm pretty sure other straight BI mods have re-used them as a normal starting faction too.