Things work differently when you configure a cluster versus when the cluster is running and needs to autoscale.

During Configuration: It only checks if the cluster's max workers <= pool max capacity.

At Runtime: When the cluster needs to autoscale, it checks if the total pool instances (idle + in-use) + the requested instances <= pool max capacity.

If the pol is full the request will fail . 

In your case the creation of the cluster should be allowed . when it has to auto scale that when u might see issues when the total instances go beyond the pool size .