Key word being "currently" ... the next version will allow revolts in minor civ territory, and yes they will spawn minor civs. I'll make it so that if the motherland is on the safe list, the rebel will be as well.
For your previous question, I can definitely make it so there's a minimum number of contacts for settling into a full civ. Seems sensible, if you're all alone then there's less impetus to pull together.
As for achieving a desired full/minor/barb split in terms of cities, there are lots of ways it could be done. I'd think the most natural way would be to raise the bar on what's necessary to make the transition to the next stage ... there's already MinPopulation setting for barb cities to have a chance to go minor (set at 2 now). Raise that to 4 and you'd see many more barb cities hang around until captured. I'll make sure the odds of settling when the city is at the MinPop are the same no matter what the value is so that things scale well.
Then, for the minor -> full transition, it's a little less obvious how to go. You could raise the bar needed to transition for the different achievement types I suppose. Adding the number of contacts variable will help a bit, as would increasing the MinorHalfLife variable (though it only applies in the new world). I could also make the half-life effect old world barb civs too ...
A % based system like you were suggesting just seems artificial to me ... it'd certainly be possible to say that, if less than 10% of world cities are barb cities, then none will form minors. Similarly, if less than 20% of all cities are in the hands of minors, don't settle. It just seems like you'd want those percentages to drop over time, and that's effectively what you'd get by slowing down the transitions.