Yes, the pool size governs all executions, regardless of the type of pipeline being executed.
As mentioned in another reply, if you enable reuse, the child pipeline to execute cannot change from one document to the next. Instead, you might try collecting all of the documents for a given child into a single document using the Group By Fields snap. The output of that snap can be sent to a PipeExec with reuse disabled and the pipeline to execute as an expression. Then, in the child pipeline, the first snap can be a JSON Splitter that breaks up the group into separate documents. This design should work in a fashion similar to what you are thinking.
Thank you, it worked 🙂
