The problem is that it is not clear what you mean by regrouping from "bottom to top". How exactly do you want to cluster your voronoi polygons? You simply want the
number of houses attribute of a cluster to add up to 600 right? The selection can be random ?
It seems that from your image you can easily do what you want to do manually - you don't need to use python, unless you have many more counties to perform the same task on.
Just select a bunch of polygons and use Vector > Analysis Tools > Basic Statistics to find the sum of the
number of houses , and then add or subtract from your selection accordingly.