Why can't 了 be used to complement 到? I have been taught that the sentence 我刚到了 (wǒ gāng dào le) is not correct because 了 cannot be used as a complement of 到. Is this right? If so, why?
我刚到了 is perfectly understandable in oral communication. Your teacher probably thinks this is verbose because:

刚 = just

了 = already

我刚到了 = I just arrived already