Can I require a PO number only when my customer selects a specific payment method? right now, If I require the PO it requires it for all payment methods I'm offering.

Unfortunately, this is not currently possible in Pressero.  The purchase order setting is a site-wide setting.  There isn't a way to make the option or the requirement based on the payment method the user chooses.
Also, refer to the articles for more details on PO Codes:  Ch. 061 Purchase Order Codes