I recently underwent a sleeve gastrectomy, and can eat no more than 150 cc at one sitting. I am also severely limited as to carbohydrates (like bread). It is therefore possible that I will never eat a whole kzayit of bread at one meal. If I make Hamotzi and have a nibble of bread, but then eat other foods (meat/vegetables, etc.) that total more than a kzayit, do I say a full birkat hamazon or borei nfashot? In such a case, does the Hamotzi still poter the other food (is the other food considered "l’lapet et hapat" if so little bread is eaten), or must I make shehakol and/or borei pri ha’etz/haadama?

According to Rav Moshe Feinstein zt"l (Igrot Moshe, Orach Chaim,Vol. 4, 41) if you ate less than a "kza'it" of bread, even if you filled yourself with other foods, you should not say Birkat Hamazon. Furthermore, according to the Magen Avraham 177:1, since you eat less than a "kza'it" of bread, the Bracha of "Hamotzi" does not exempt the other foods from their specific Bracha. May Hashem grant you a "refuah shleima".