In Final Fantasy VII Rebirth, fans will be introduced to beloved characters Cid and Vincent. However, according to director Naoki Hamaguchi, players won't be able to control them in combat encounters. In an interview, Hamaguchi explained that although Cid and Vincent will accompany the party on their journey and provide assistance, they won't be featured as playable guest characters in battles.
They're going to appear in various parts of the story and scenarios where they play a role or they'll help you out or assist you in some way, but they're not actually fighting as guest characters in that sense
This decision was made to ensure that the third and final game in the remake trilogy will bring new surprises and experiences to players. Hamaguchi emphasized the importance of saving Cid and Vincent's combat abilities for the later installment. While they won't be involved in fights, both characters will still have significant roles in the game's story.
If we used up all of the playable characters in the second part, then we would lose out on quite a lot of new surprises and new experiences for the player in the third game
Players will have the opportunity to delve deeper into the characters of Cid and Vincent through side quests, where they will be the main focus. These quests will provide a chance to learn more about their backgrounds and motivations, enriching the overall experience of Final Fantasy VII Rebirth.
Source: ign.com