Personally I don't think Pybal should be rejecting that; it's a valid configuration from a technical standpoint, and there can be valid reasons to have it, at least temporarily. But we may decide that in our specific environment that should be avoided at all cost, so perhaps that logic should be implemented elsewhere - in the code that manages pooling state.
This is confusing to me, as I've heard things from @Joe and a couple others indicating that they thought Pybal already worked the way that the task title describes.
Is the min-pooled threshold 1? So that it would reject a configuration with nothing at all pooled, but 1 node pooled is OK?