Hadn't seen your other comment about 300 sized pool. I agree that I wouldn't call 300 vm pool small, but would like to know what you meant by "chaotic". Was it as unbalanced as a 80 vm pool? Also do all the datastores have the same size/freespace to begin with?
I believe reducing the pool size starts by removing the lower numbered vms. Now if all the initial vms get placed on the same datastore then you wouldn't like the results of reducing the pool size. I would recommend trying out with the lowest concurrency in 5.1.