"My girlfriend's eyes are black and beautiful."
i've been taught that the words on either side of 又 should be the same syllable, does anyone else agree?
That would be a good literary technique in Chinese. We have an adjective 鏗鏘 / 铿锵 (clangorous/ sonorous), which is describing sentences like these. With the same number of syllables on each side, there would be a clear symmetry of rhythm. If the same vowels are used, then it would sound even more beautiful. For example, in the above sentence, I can make it sound beautiful by saying 我的女朋友的眼睛又黑又美。
Having said that, it is a technique, not a rule. We often use different syllables when we are describing things without making an effort to make the sentence "beautiful".