1. 留学生 is more formal than 外国学生。Especially when it comes to the expression used in public. For example, the dormitory for foreign students is called 留学生宿舍。
2. 他认识了一个漂亮的姑娘 is the right sentence. If it becomes '他认识了挺/很漂亮的一个故娘'，then it can be used on some casual occasions.
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