Indeed... Square say that sorceresses have normal human lifespans; this is canonical info from the Ultimania guidebook. Ultimecia lives "many generations" in the future, to quote the game, so unless she's using Secret Longevity Magic or somesuch, there's no realistic way for the two to be the same person.
The game definitely plays with the risk of Rinoa 'turning evil' like some sorceresses before her; she's afraid of becoming "the world's enemy" like Ultimecia. She also knows that Ultimecia will one day inherit her powers, several generations along down the line. In that way, Rinoa and Ultimecia are 'the same' as each other, though they're not actually the same person.