I never said she used any other GFs. I said she used at least one GF that we know of.

My take on the Griever thing is that when Squall was killed, Rinoa blocked his path to the afterlife and imprisoned it in the Griever ring--something had had sentimental value to him--where over time he was transformed into "the most powerful GF in Squall's mind." He had become what he had imagined the legendary Griever would have been like. Thus, the GF that he had become was the most powerful GF "in his mind," i.e. he transformed into what he had imagined all of his life. This gives Ultimecia's "juntioning unto Griever" so much more meaning.