Is it permissible for a worshiper to transition individually from praying alone to praying behind someone else as a follower, or to inform the other worshiper that the prayer is not obligatory by saying "Subhan Allah" before he says the "takbir" with him, and what is the ruling on the prayer of each of them in this case?

It is permissible for someone performing an obligatory prayer to follow someone performing a supererogatory prayer. There is no need to alert the Imam by saying "Subhanallah" (glorified is Allah) or to inform him after the prayer that you were performing a supererogatory prayer. Both your prayers are valid. Your moving after you finished your supererogatory prayer to become a follower (ma'mum) to the right of the person who was already a follower is not preferable, but the prayer is valid.