I always assumed that the Terra WAS riding around in a Reaper. The "Predator" was just the devs (understandable) inability to adequately represent Amano's art in animated sprite form. (Still image monster sprites tend to be more faithfully rendered.) The same reason why the character sprites tend to look so little like Amano's work.

Basically, saying that Terra's armor is different from magitek armor is like saying Terra herself is a different character than her portrait.

Does FFVI itself ever specifically state that the armors Terra, Biggs, and Wedge were riding around in were a different model than the ones you fight?