If the Tyrells had joined the rebels during Roberts Rebellion how would the war have been different?

Well, that would be wildly unlikely, as the Tyrells don’t have a motive. But if they did, it transforms the war immensely: the Battle of Ashford and the Siege of Storm’s End don’t happen; instead Robert links up with the Tyrell forces after Summerhall, most likely defeats Connington decisively given his much larger than OTL numbers. With the changed geostrategic picture, I don’t know if the Trident still happens, but almost certainly King’s Landing is put under siege before the Lannisters arrive.