Workflow user group is a segregated hierarchy.
For example, my user has an approval limit of 1 for purchases in the approval user setup. If I setup an approval workflow that uses a workflow group that contains my user and my user submits an approval request the approval is instantly accepted. It has bypassed the approval user setup because I told it to.
Your challenge is more to do with you'll need an approval workflow per PO logger. Each approval workflow uses just one group. You can get a different workflow approval to be used if you have a filter in the header of the document. For example the "Assigned user ID" or the "Purchaser Code".
