The FX-9 receiver is wider than a standard AR-15 receiver in a couple areas that interfere. It would fit extended, but may not fit collapsed.
This will not work properly with the MPX. You will want to use a model meant specifically for the Sig Sauer MPX.
The PDW is not compatible with a folding stock adapater.
The Strike Industries PDW 2-Position Stabilizer Brace would fit your 300 Blackout AR, however, the buffer system was optimized for use with 5.56/.223. Using the system with 300 Blackout may require you to tune the recoil system to ensure proper functionality.
The Strike Industries PDW 2-Position Stabilizer Brace is not compatible with large frame AR's (LR308, AR10, etc) but will work with AR15 pattern rifles chambered in 5.56, 300 Blackout, 7.62x39 etc.
The Strike Industries PDW 2-Position Stabilizer Brace is compatible with the CMMG Banshee, you will need to use the Strike Industires AR PDW 9mm buffer (linked below).
https://www.opticsplanet.com/strike-industries-ar-pdw-9mm-buffer.html
This stock is not compatible with the Ruger PC Carbine or charger.
There is another QD attachment point under the removable cheek piece at the end of the buffer tube.
The Strike Industries PDW 2-Position Stabilizer Brace might work with that set up depending on what adapter you are using. Please email us at techsales@opticsplanet.com for more information.
The Strike Industries PDW 2-Position Stabilizer Brace will work with a PSA AR-V 9mm.