Re: Selfish people in the dojo
in our dojo there are some unspoken rules everyone complies with. When there is an absolute beginner, this person will have the whole lesson some hakama partner, generally a yudansha.
Sometimes the teacher says, if we are doing some advanced techniques that require a bit more dynamic movement, that hakama people should train together. That wouldn't be more than maybe 5 % of the total time. In general we are encouraged to mix. Once train with a hakama, next time with a white belt, then with a hakama etc., and if there are not too many people, everyone tries to have trained at least once with every single person.
I'd say normally people should see the interest in training with everyone themselves, but if they don't, it would be the dojocho's or teacher' role to remind them.