The barrel plug on the holster is only important if you want to run a gun without a weapon light in a holster built for a weapon light. The ALS mechanism still grabs the ejection port to secure the gun. The barrel plug just indexes the gun to prevent it from rotating out of alignment when the light isn't installed to fill the void meant for the weapon light.
You can remove the plug. I do on all of my ALS holsters intended for threaded barrels. If you keep the weapon light installed, you're good to go. I replace the block that formed the barrel plug with plastic washers to keep the holster supported at the correct width when the block is removed and the retaining screw is reinstalled. Here is one of my holsters to give you an idea.
You can buy the standard G17 holster and remove the plug once you get the threaded barrel. You can buy the G34 holster and try it with the threaded barrel. It will work for the G17 since the dimensions are the same at the ejection port. The standard G17 barrel just won't reach the barrel plug. The threaded G17 barrel may index on the barrel plug, but if you try it and you find it doesn't because it's too long, you can always remove the barrel plug.