There was an earlier thread that discuss this topic and someone post an link to a website which give a in depth anaylsis on this theory until the last line of the page was something like this:

"So just who is Ultimecia? We decided to ask the big boys in japan and they reply, No, Rinoa is not Ultimecia. What an easy way to end the debate"

The "big boys" should refer to the square's staff.
Do anyone know or remember this website? It also have other FF series analysis.