You could start both of them in Aesop B, no problem. Since your older is in 6th grade, it would make more sense to start in Aesop B. We recommend taking 1 year with Aesop B and supplementing with CW Poetry for beginners for about 12 weeks. You could start that with Workbook A for Poetry, not Workbook B, since they build on each other.
Rod and Staff is a fine program for grammar as is Harvey’s. You should not have any trouble using that. In the Homer Core book we do leave blank spaces for you to add i n your own grammar text. Every lesson notes what its about and there is a list of grammar concepts to cover, you just cover those concepts in your own grammar text.
5th, 7th, and 9th graders
I would start them all off in Aesop, if you're using the workbooks I'd use workbook B. I'd go through B as quickly as possible, making sure they knew the material of course, with the older ones and then move them to Homer.
It is also recommended that the 9th grader go through Homer as quickly as he can and be moved to Diogenes.
If the 9th grader has a solid grammar background, you may want to do him separately starting in Homer A.
12th, 11th, 10th and 8th graders
With 4 kids in the upper grades you are going to span a great deal of material, especially to get the 12th grade up to snuff. You have to decide where you want to put in your efforts, whether you want to teach one or more than one level and go from there.
IF their grammar is solid and ---- if you want to teach two levels, I'd take the 12th and 10th grader directly into CW Diogenes- Maxim to work as much as possible on the essay work. I'd work that book through in a year or faster if they can. The Student Guide to CW- Diogenes Maxim will be out this summer prior to the fall semester start.
I'd take the 10th and 8th graders (assuming their grammar is solid) into CW Homer and work them through. You have more time with the two younger students, so you have more time to lay a good foundation. I'd take them through CW Homer Workbook A and then B in one year, and then do Diogenes: Maxim the following year.
Two students 7th and 8th grade
8th and 7th grade, he's a reluctant writer and she loves to write.
Start them both in CW Aesop workbook B, do the beginning poetry workbook A and then start on CW Homer.