the chazon ish had a shitah that you cant follow a halachah if you were m’ayain properly and disagree. He holds that you cant use any metal, even nails, to keep a succah together, even though there is a Ritva that says b’feirush that you can, and the bais yosef paskens like that ritva. i assume R’ Moshe had this shitah to some extent as well, but this isnt a universal shitah, and even if you do hold of it, it would only matter if you looked into the sugya properly, which i doubt anyone except a gadol/posek could be sure of – meaning pashtus anyone reading this is not enough of a talmid chacham to decide thathe is 100% sure that a rishon or acharon is wrong. be safe, ask your posek what to do if a shailah comes up, definitely dont argue with rishonim.
Avi K: the Gra, according to many gedolim, was on the level of rishonim. a certain gadol said he was on the Rashba’s level of learning, so thats not a ra’aya at all