It's already been confirmed in an interivew.
"The original party members are all present in Final Fantasy VII Rebirth. In the previous title, Final Fantasy VII Remake, Red XIII became an accompanying member in the second half of the game, but he will become an official, playable party member starting from Final Fantasy VII Rebirth. Similarly, there are characters who are accompanying members in Final Fantasy VII Rebirth, that will become official party members in the next title."
Cid and Vincent will mostly likely be playable in the final installment and Red XIII and Cait Sith are this game's new additions. 2 new characters per game