However, I did notice that in general, when pairing up to train, folks didn't mix up very much; they tended to train within their rank groupings more or less and this was easy to see because they used a colored-belt system. Is this the way it usually works in sit-by-rank dojos?
Sensei normally encourages the Yundasha to practice with the beginners at the beginning of each practice, and often starts with basic variations the beginners should learn. Then the Yundasha and other experienced students can assist the beginners to learn.
Sometimes, we later move on to more advanced techniques and split the group according to level, the same is done with Randori for people who are not yet experienced enough for it and for weapons practice (particularly weapons Randori and Kata).
The idea is to balance the contribution of the advanced student to the beginners and his own studies.