Then I had the idea, since we school year round anyway, and my kids are placed well so don't need to go half speed (what we generally do when we do DITHOR with the guides).... why not rotate? So if anyone could tell me why this would be a bad idea, I'd really appreciate it.

FWIW, I consider Bigger to be a great 3rd grade program and CTC to be 5th... so in that respect my kids are 'ahead'.
So my plan is, when Jayden finishes up Preparing, instead of jumping right in to CTC we will do a "unit" of DITHOR. While we are doing that we'll continue math, English, and dictation (because it dawned on me that I don't have to keep those subjects exactly with everything in the guides

That should take us about 18 months to get through the 9 genres in DITHOR, which is what it is scheduled for in the older guides and I'm ok with that for my younger ones as well. It should also work well that we'll be starting over about the time that Julianna is starting DITHOR.
It will also have us finishing a guide in a full year, which is what we do anyway for the most part... give or take.