if according to rashi in ela r' yehoshua hee the day for resting the lulav is the 7th day, then that is also a siman klalla because we are in the succa on the 7th day so you shouldnt be able to start saying mashiv haruach at that time and it seems like this is talking about the r' yehoshua of the mishna
ari, west hempstead NY
You could have asked the same question on Daf 2b where, with reference to Rebbi Yehoshua's opinion, Rashi, DH mi'Sha'as Hanachaso' says the same thing as he says later on Daf 3a (the subject of your query).
If you look there at the notes of Maharav Renshberg, you will see that he deals your problem, and concludes that although Rashi talks about the seventh day, he agrees that, according to Rebbi Yehoshua, one recites 'Mashiv ha'Ru'ach' only on the eighth , as he specifically says in our Mishnah.
Be'Virchas Kol Tuv
Eliezer Chrysler