Hmm, but if you allow this much granulometry, you wlil have to be playing with allocation sliders all day.
I think it should be mostly automatic. The amount of different resources your colonies produce/can use, and the distance/trade it's getting from other planets and colonies, determines automatically what sort of fleets it spawns and how many (in addition to modifications by that planet's infrastructure)
So if you nuke a lot of faction's planets and the remaining ones cant get trade resources from some other place, they'll be able to spawn a lot less ships.