I apologize if this is already answered elsewhere, but ...

Is it that movement, ranged combat, and close combat formations must all be "faction-pure," or can any units on my team join any formation, except mercenaries?

Formations can only consist of units from the same faction. As for
Mercs, I'm hoping WK will come up with a rule, soon, that allows
them to form up (regimentally). In other words, Eridani Light
Horse with Eridani Light Horse, Wolf's Dragoons with Wolf's
Dragoons, etc. ;)