There have been some changes in their approach to this in the last few years, and I have the impression they are tightening up. I think where you have to focus is here, from the non-commercial regulations:
"The purpose of the motorized watercraft must be to carry gear and provide support. The hybrid option is not available if the trip's non-motorized craft are capable of safely hauling all of the trip's gear"
I have no way of knowing if your oar rigs would -- in the eyes of the Park and in terms of size and capacity -- be incapable of "safely hauling" all the stuff you plan to bring. And I don't know how they make that determination, either. I have never heard of a numerical boat limit, but that doesn't mean they don't have something like that as an internal discriminator.
(I mean, if kayakers can self-support down there, what raft or combination of rafts would really need a motor rig for support? I think it's a sort of unworkable characterization, but there it is.)
My best advice would be to call the River Office again, and talk to someone some more about it before you send in your permit application. The folks there are very good with "customer service". Give them an accurate description of your actual boats. That way, if they issue you a hybrid permit, you don't have ranger issues at the Ferry. And if they say your proposed fleet doesn't qualify for hybrid status, you can recalibrate at home, rather than on the ramp.
Hope this helps.