Re: Sempai-Kohai relationship
We have a relatively informal sempai/kohai relationship but it certainly does exist even without the use of names. As the senior student at our dojo, I usually tell my junior students that they will eventually gain an understanding of where they stand and who should attack first.
That said, the lines are not always clear-cut.
I took a class last week that included a student who started several years before me, but due to a stop-start approach to his training and long absences in between training I am yudansha and he is still a mid-level kyu grade.
I certainly consider myself to be senior in terms of ability and understanding (I've also trained for nearly 25 years in other martial arts apart from Aikido whereas this student has only studied Aikido), but am not so hung up on the concept to try and force this student to conform to a notion of sempai/kohai.
When we train at our national schools or special days of training there are certain people that I have no problems whatsoever in deferring to as their understanding eclipses my own, but those same people are probably the last ones to try and enforce the sempai/kohai relationship.
Personally I wouldn't get too worried about the notional relationship and simply concentrate on the training. It is true that you can learn from anyone and everyone and that the idea of someone being senior to you simply means they are in a different part of the same path - it hardly matters whether they are in front, beside or behind you, you are all pushing toward the same end.