I cannot comment if the 18 PSI line pressure is appropriate, per se, for your pouch integrity tests. I can, however, direct to several industry standard tests for whole package integrity evaluation.

The positive pressurization of packages for leak detection such as the testing performed per ASTM F 2096 “Detecting Gross Leaks in Medical Packaging by Internal Pressurization (Bubble Leak Test)” has as part of the procedure a calibration step, which is essentially conducted by making a known defect in the pouch with a calibrated wire of calibrated diameter and slowly increasing the pressure until the know defect begins to leak.  The pressure level at which the known defect begins to leak is the test pressure. It is different for every package of every different type of geometry and size, hence no pressurization standard. If your package fails or bursts at or about the required test pressure then the test method may be inappropriate for that package design (assuming that seal strength is sufficient for maintaining physical integrity, as determined by other means) and you need to select a different test method, possibly a vacuum leak test.

According to ASTM F2097 “Standard Guide for Design and Evaluation of Primary Flexible Packaging for Medical Products”  under Package Integrity (the ASTM test methods for Package Integrity), perhaps trying a vacuum leak test such as ASTM D 3078 “Determination of Leak in Flexible Packaging by Bubble Emission” may be a viable alternative to prevent bursting of the new pouch.

I hope this information is useful, essentially there is no absolute minimum pressurization standard, other than the necessary test pressure as determined through those test methods utilizing positive pressure and having a test pressure “calibration” step.