Generally speaking, there is no limit. You can take anything you like. But if you like being a teacher, you might as well take the postgraduate exam in Chinese education, or you can take the postgraduate exam in Chinese major, such as ancient literature, modern and contemporary literature, foreign literature, modern Chinese, ancient Chinese and so on. No problem. As to whether it is difficult or easy, it depends on the individual. Some people find it difficult, others find it easy.