[NTREEG]

I'm just a student as well and can't tell you with any certainty that it's wrong. But whenever I've accidentally used でした　with an i-adjective, I was told it was wrong and had to change it to かったです.

Of course there are adjectives that end in "i" that are actually na-adjectives which you would use でした with.
Ex:
町がきれいでした。
両親が元気でした

If I remember to do so, I'll ask my teacher tomorrow to see if I can get you a more definite answer.