Does anyone know if there will be some kind of bestiary with the source book? Or any ideas about this? How will this be handled?

Until there is one, or at least an answer if there'll be one- there is our unofficial one here...

There has never been a bestiary released for the Star Wars line, FFG have stuck closely to their plan of only printing books with material for everyone at the game table. But Genesys brings a new dawn on a new day, they may change their plans with this project. Considering the Terrinoth book has more pages (just) than the core book I would expect a large selection of NPC’s throughout.