You're welcome! I love spreading around my silencer addiction! And don't worry, your barrel isn't a problem at all. The normal 9mm thread pattern is 1/2x28, but lots of factory barrels (SIG, H&K, Glock) come in metric 13.5x1mm left-hand threads (which is what you have). So all you need is a 13.5x1mm LH piston instead of a 1/2x28 piston and you're good to go (I've never seen a modern silencer company that didn't offer 9mm pistons in both those thread patterns).
Some people prefer the left-hand metric threads: they tend to be less likely to come unscrewed during shooting (which can cause baffle strikes) because the metric pistons from most major companies usually use o-rings and the standard ones usually don't.
You can order your silencer with the correct piston for your barrel. But if you end up with a silencer that has the wrong piston, you can either just buy an extra piston or trade it out with someone else. At the FFL/SOT I used work at, we would trade out pistons free of charge if a customer bought a silencer and wanted a piston with a different thread pattern.