simcha613
A better example is R’ Moshe’s Teshuva on peanuts where he says they arent included in kitniyos for the very reason you mention. However klal yisroel has accepted that peanuts are kitniyos (I’m sure their are yechidim who do eat peanuts but clearly the general hanhaga is not to)
On the other hand Corn which wasnt included in the original geziera is kitniyos according to all (as far as I am aware). As mentioned, some tried to include Potatoes but the practice is not to.
The bottom line is, their isnt always logic to these minhagim and the “reasons” given often lead to more questions than answers . Consider we treat kitniyos more chamur than wheat, since we DO eat wheat (As long as follows strict rules) yet rice we dont eat at all. They develop out of love for mitzvos and concern for issurim. There is no need for you to add new issurim just follow the practice of klal yisroel, maaminim benei maaminim and you will be fine.