I’ve been running a 9mm buffer in a 10.5" 5.56 SBR with good results.
Would this idea of running a 9mm continue to to transfer to a 7.5 barrel with a Noveske Pig?
The 9mm buffer doesn’t have the sliding weights inside that make it a buffer. To make a buffer with sliding weights as heavy requires tungsten, tungsten is expensive. The 9mm buffer is a cheaper way to get the needed weight but is a poor choice for a gas operated AR due to bolt bounce.
As to what buffer weight to use, you should ask the barrel maker. The gas port size could be designed for an H3 buffer, or one of those pistol systems with a 1911 spring and almost no buffer at all.