I received a Purchase Order (PO) instead of a Subcontract Agreement (SA) . Do I lose any legal rights as a subcontractor

Good afternoon,

The form isn't necessarily an issue itself --- it is the content and substance of the purchase order and, as you say, the "long list of qualifications." What are the terms and conditions of the arrangement? That is what is important.  

You would be surprised at how little it requires to have a valid written contract. As long as the necessary terms: scope of work, price, time, and/or other factors that spells out what the terms are in place, whatever documents that are provided to you MAY be enough to form a binding contract.  

And if you contract direct to owner, or have provided a pre-lien notice to owner, then you may have the basis for a mech lien.