According to the Jewish Hashakafa, could certain ideas that were expressed by Rishonim, rejected by Achronim. For example there are many times were in classical writtings it’s stated that one shouldn’t dress or act like Goyim. however, Rabbi Samson Hircsh clearly writes that one could wear what everyone else wears?
The Psakim of the Achronim are all based on Halachic precedents set by the Rishonim. A Psak not based on Halachic precedent is valueless and would be rejected by other Halachic authorities. In regard to the example you bring - the major sources among the Rishonim are Rambam (Avodat Kochavim 11,1) and Shu”t Maharik (responsa 88). As decided in the Shulchan Aruch (Yoreh Deah 178,1), one may dress in a way similar to that of non-Jews if the fashion is modest and not connected to a non-Jewish form of worship. The Vilna Gaon forbids attire similar to that of the gentiles unless it is the kind of garment we would have worn even if it were not the fashion among the gentiles.