Hello, Mayland! Long time, no talk.
I wish our dojo had your problem; lots of students to deal with. Unfortunately, we're at the other end of the member spectrum. However, if I may, I'd like to mention a few tips from my own teaching experience, if you don't mind.
1st - using senior students to teach junior ones - that's ideal, but it has to be managed right. I'd recommend giving the job of teaching newcomers (as opposed to 'beginners'; I'll arbitrarily set the separation at about 10 classes - just long enough to be able to take basic ukemi without killing themselves) to 2nd-kyus, and beginners to 3rd kyus. Reason: The first few lessons are critical in any skill; you need someone with coaching experience to bring them into the fold correctly. Once they've got their feet, as it were, you can give the job to the less experienced 3rd kyus - gives them excellent teaching experience and they should be easily good enough to teach begginner skills.
2nd - rotating instructors - OK for intermediate students, but bad for beginners and newcomers. Reason: Different people have different training styles and skills; too much difference will confuse and distract very low-level students, limiting their learning. I know, this sounds like a minor point but it isn't - we all have experienced this at one time or another, I'm sure. So; while rotation is good for intermediates, try to keep newcomers with one or two good instructors.
3rd - communication. Again, most important for instructors working with the newer students. If a different instructor is teaching them, make sure he or she knows a)what they covered last class, b)how far the students had proceeded during the class (i.e. they were learning ukemi, got as far as a forward roll from the knees, so its a good idea not to work on standing back rolls yet) and c)what you intend to accomplish in this class.
4th - whenever possible, with newcomers, its a good idea for instructors to work in pairs - one teaching and observing the students, one observing and correcting the students. Note the overlap. I'm sure experienced Senseis will agree - you can't watch newcomers hard enough. (lol!)
Anyway, these are general principles; I don't know enough about Aikido to consider it specifically, but these points should help organize things a bit.