1) It may be true the the words of the Behag carry a certain added authority over those of the Rishonim. They are referred to by the Rishonim as Divrei Kabalah (i.e. words that are considered an authentic transmission of the Torah of the Ge'onim) (see Tosfos, Chulin 44a, D"H Kedei), but his opinions are not considered beyond challenge by the Rishonim (see Tosfos, ibid). The Rishonim - and everyone else - are only limited from arguing with Halachic rulings in the Talmud itself.

2) I don't know if Rashi personally did not eat Kitniyos, but the Minhag certainly existed among the Ashkenazim in his times. See Sefer Mitzvos Katan (Mitzvah 222, Haga'os Rabbeinu Peretz) who says that already in his time it was an ancient Minhag, but that his Rebbi - Rabbeinu Yechiel, who was Ashkenazi - ate beans on Pesach. So it is not clear that the Minhag was universally accepted among the Ashkenazim.

Rabbeinu Peretz (ibid) says that there were four things included in the original Gezeirah made by the Ashkenazi Rabbanim - white beans, peas, rice, and lentils. Chumus was cultivated in European countries from very early times and is referred to variously as a pea (chickpea) or a bean (garbanzo bean), so I think it is quite likely that it was included in the original Takanah. Therefore if Rashi had a Minhag to not Kitniyos he probably didn't eat chumus either.

Kol Tuv,

Yonasan Sigler

This is not a Psak Halachah