1) While practising techniques, I'm sure it's much better in terms of progress for the beginner to work with higher rank students. Therefore, dividing groups into beginners and advanced is unfortunate.
In any sport or skill, be it football or pottery, - you need a _better_ opponent/colleague/team mate if you want to learn, to progress. If Maradona or Ronaldo spends 10 years playing football with kids - most probably their own level will fade.
And here a dilemma arises for high ranked students - if I want to progress myself, I should stick to students with higher rank than mine. If I want to help the dojo and Aikido as a movement - I should spend more time with beginners. Ego may influence the mind, on this stage, but actually we are learning how to control our egos through ukemi
2) While practising ukemi falls - I'm sure a dedicated class for beginners is very useful. It takes years to master ukemi, and many dojos don't pay much attention to it, in my experience.
If you introduce a 30 minute ukemi practice section in your advanced/regular class - many yudansha will be bored and pissed off. Therefore the only choice is to make a separate class for that.
Since you don't need a partner to practice ukemi (often) - it doesn't break any of the ideas mentioned in the first block. You don't really divide class.