We just don't know at this point. It does make sense that there are other versions of Sage mode.
Since Sage mode appears to require a rare kind of student, and it seems that the person must be picked by their respective summon to learn the technique, it would be understandable that very few people know about it. Furthermore, most people who try to learn Sage mode are either rejected or fail miserably (the toad statues and Orochimaru are evidence of this).