One of many possible word orders. In this particular sentence, the words can be rearranged to any order.
The first-person pronoun kú is implied by the agreement suffix on the verb kí, so it can be omitted.
If it's clear from context that we're talking about food, the object can be omitted altogether.
Úni mumwe is the noun phrase here; if úni was modifying the verb, it would be kíku úni nga mumwe.
*Úni iki úni mumwe would be double negation, so nga is required here.