意外に高いです is also correct. に is usually used for adverbial usage of na-adjectives, i.e, not only when with i-adjectives but also when with na-adjectives or verbs. と is basically for -tari conjugation type adjectives （タリ活用形容動詞） like 平然, 悠然, 厳然, 堂々, 粛々,,,. 意外と is an exception.
It is important, though, not to think of it in terms of "what goes between a -na adjective and an -i adjective" because there will be times when it is simply an "and" sort of situation and "de" (-te form of -na) is used.