I like it. I like it a lot. I had often wondered about the similarities between T/Y and S/L. So, wait, first, were the similarities between Yuna and Lenne a coincidence?

Shuyin died a thousand years ago trying to usurp control of Vegnagun to save his love, Lenne, for she was soon to Pilgrimage for Aeons to battle Bevelle. They, in the end, died together, but Shuyin was unable to accept it. Filled with bitterness, his essence merged with the pyreflies which preserved his memories (along with his desire to use Vegnagun on the world) for a thousand years, at some point being present in the Mushroom Rock cave.

Tidus is a dream of the Fayth who wished to defeat Sin. As is common in any dream, Tidus and the rest of Zanarkand was based on the physical recollection of what had once already existed. However, again, as is common in dreams, Tidus and Zanarkand were not identical to whom or what they were modelled after. Tidus's thoughts, emotions, and memories, his ideals and opinions were all his own. His body possessed the memories present in the body of Shuyin thus allowing him to learn quickly to fight. However, the memory of the body is different from the memory of the mind. Anyone who has ever watched the Borne Identity or The Long Kiss Goodnight would know this.

The physical memory is how Tidus' attacks look similar to Shuyin's as has been mentioned. Easiest question ever to answer is why their attacks don't simply share their names as well. Tidus names his attacks based on his attitude whereas Shuyin named his attacks long ago based on his own attitude. This in itself lends evidence that Tidus does not think like Shuyin.

A question I developed when reading about Dream Zanarkand is why it too was destroyed. Ten years previous, Jecht, an unbelievable Blitzball champion, was brought to Spira to be Summoner Braska's Guardian. Perhaps after merging with Yu Yevon in the casting of the Final Aeon, his thoughts concerning Tidus drove Yu Yevon to destroy Dream Zanarkand in effort to prevent Tidus from becoming a Guardian to a future Summoner. Perhaps it was somehow known that Tidus would stop the cycle of Final Aeon and Sin as in some prophetic vision.

I can't really make any real assumptions, I haven't played either of the games in a while .