Re: who can wear hakama
The hakama question has numerous conflicts. When I explore it, I see many photographs showing O'Sensei teaching a roomful of students, with almost nobody wearing hakama, even black belts. In other photos, everybody has one. Saotome Sensei tells a story from his uchideshi days about being dismissed from the mat by O'Sensei because he had forgotten his hakama. "What makes you think you can receive your teacher's instruction in your underwear?" O'Sensei asked. It also seems in those days that O'Sensei wasn't picky about styles, grades or colors of hakama. Saotome Sensei says that many students showed up for practice having snuck their fathers' expensive silk decorative hakama out of the house.
One thing does seem certain. That is that the hakama, in Aikido at least, was not intended to be one the trappings of rank, although modern interpretations seem to have made it so. It is realistic to assume, since a good hakama is expensive, a student should be given the time and opportunity to gauge his commitment to the art before having to invest in one. In the present, it seems to be basically up to the respective organizations, and even to individual teachers, to decide how to treat it. In gatherings of mixed styles, or when visiting a dojo not of their own, I have not personally seen any student who didn't have one asked to put one on, nor have I seen any student who wore one asked to take it off. Your mileage may vary.