anda should also be acceptable as it is a more polite/formal form of kamu
agree! kalian also.
Shouldn't "Kamu dan dia pikir." be accepted as well?
"pikir" should followed by other word, i think..
"kami berpikir. = kami sedang berpikir".
(i) kami pikir baju ini bagus ✔️
(ii) kami berpikir baju ini bagus ✔️
both (i) and (ii) have same meaning, actually, and you can use them in convo.