My two cents;
The shach argues regularly on Rishonim, I can think of about ten places offhand. However, he is very reluctant to argue against the Bahag and the others of thart time, because they were Geonim, and they did not write their own thoughts, rather they were actually quoting Ravina and Rav Ashi (as the Shach puts it; “kol divreihem divrei kabalka”). So there is a difference between the rishonim and the Geonim.
The one who said the Gra was like the Rashba was Rav Chaim Velozoner in the Hakdamak to Sifra Dtzniusa, not the Gra himself.
According to the Rambam in the Hakdamah to the Yad, you can argue on anyone until the Gemarah (possibly this includes the Geonim, I don’t know), but as all of us know, you would need to be either very sure or very foolish to do so.
P.s.; the Chazon Ish with regard to sukkah is not arguing on anyone; he is rather quoting a minhag in Lita to be machmir for the shita of the ran and ramban AND RITVA that the maimod needs to be of something kosher for schach. So that was a misquote. However the Chazon Ish does argue on Rishnim, I can remeber one chazon ish in kilayim who argues head-on with a Ritva