The chances of you transforming into Anti-Sora on your first transformation is very low. If you keep abusing your Valor, Wisdom, Master, and Final forms repeatingly, then your chances will increase. But no, you can never turn INTO anti-sora for the rest of the game, unless theres some hidden cheat codes in AR Max that allow you to do so.
I almost never use magic except when I cast it for entertainment. The only ones I realy use are Reflect and Magnega, but they make the game much too simple.