What is exact meaning of きっと良い? The ～良い part is easy to understand, it means "good". But きっと～ part is not very clear to me. Also, could this phrase be used in official language?
This is what the Wisdom Japanese-English Dictionary says:
きっと 〖確かに〗surely, certainly; 【きっと…する】be sure [certain] to 〘do〙; 〖間違いなく〗without fail（必ず⇨①）〖…に違いない〗must 〘do〙 (!do は通例状態を表す動詞) ; 〖十中八九〗probably（⇨多分）.
In official language usually more elaborate phrases are used, but I am not completely sure about the usage of this one in any more or less official papers.