This pretty much. People tend to field the primary from the supplement then take formations from the main book.Formations take the rules base from the codex or supplement they come from. If it's a supplement, then refer back to the base rules for any specials they can have. You can take any formation with any appropriate faction/matrix legal force, but they don't take on the "aegis" of the main formation, they carry their own. Just as a Mechanicus force doesn't put it's rules on an allied Sister's battalion, the Farsight enclaves don't impose their rules on any other of THEIR faction's formations.
It's no different Dark Eldar allying in from Coven Supplement really.