If you're gonna do a Terra is Xehanort theory do the fusion theory because that seems to be the popular theory going around. I personally don't believe Terra to be part of Xehanort at all because all fusions have in the games have shown evidence to the contrary of what people state the fusion theory is all about.
What fusions? The only ones I can think of are when Namine and Roxas returned to their somebodies, and that doesn't even count. =/