Rouge's requirement for Lucifer to be able to join their ranks was for Lucifer to pass the five exams that he will personally set for Lucifer.
Of course, those five exams were all different and each one of them were testing the different abilities of Lucifer.
In fact, the exams that Rouge set up for Lucifer were so difficult that the exams that the others took earlier wouldn't even be able to compare.
And Rouge deliberately set it up that way since he could feel that Lucifer was stronger than the norm.
Well, if Lucifer was not stronger than the norm, Rouge would definitely not even let Lucifer take the exam.
Hell, he would have killed Lucifer straight away if Lucifer was not any special.
After all, there was a proper time and place for these exams, and Lucifer was the one who didn't take the exam at the proper time.