Not exactly sure what you are asking: kuwa behaves like other monosyllabic verbs in all tenses (I can think of) except the present: alikuwa, amekuwa, atakuwa, angekuwa, angalikuwa with the negations hakuwa, hajawa, hatakuwa, asingekuwa, asingalikuwa. Subjunctive is niwe, uwe, awe, tuwe, m(u)we, wawe. Neg. subj. is nisiwe, usiwe, asiwe, tusiwe, msiwe, wasiwe. Kuwa! is the imperative, but typically the subjunctive uwe is used (2nd person singular).
The present of "kuwa" is a subject with several interesting aspects, but you're right, often "ni" is used for all persons, and the forms (ninakuwa, unakuwa, anakuwa ...) are rarely if ever used. To be in a place (present tense): niko, uko, yuko, tuko, mko, wako and -po or -mo could take the place of -ko here. There's more that could be discussed, but hopefully this goes some way towards answering your question.