Kuri you have it backwards, the ShinRa family of Gaia (FFVII) are descended from Shinra of Spira (FFX-2), Spira came before Gaia, the ShinRa family are far descendants of the Al Bhed... Shinra from Spira designed the technology to harvest the energy of the Farplane as a source of power, the same tech used later by ShinRa Electric Power Company to harvest the power of the Lifestream in Gaia...

Sephiroth was the biggest threat simply because he could control Jenova, though the cause of that is due to Hojo's meddeling with his DNA in utero...