I have no personal experience with the Optimus but have been researching it as well.
It comes with a 1/2x28 piston. This is for pistol use. A booster piston, along with its spring, allows the suppressor to recoil, while recoiling it "decouples" its mass from the end of the barrel, allowing the slide to cycle. If it wasn't for this, a barrel with a suppressor hanging off the end would weigh to much and wouldn't unlock from the slide during normal fire.
Running the piston on a fixed barrel which doesn't unlock from the action will put to much force on the supressor causing it to beat itself to death. So no, do not use the piston on a rifle. You can get fixed spacers to replace the spring on some supressors, but with the optimus you should do 1 of 2 things for rifle use:
Buy a direct thread adapter for fixed barrels in your pitch of choice (1/2x28 in your case, this will work great for sub guns thread 1/2x28 as well)
http://www.silencershop.com/accessories/rifle-silencer-accessories/griffin-taper-mount-direct-thread-adapter.html
Or, what I would recommend, Just buy 1/2x28 brake/flash hider adapters for all your rifles and enjoy the QD nature of the taper mount system..
http://www.silencershop.com/accessories/flash-hiders/griffin-armament-taper-mount.html
Also, yes, the 1/2x28 piston will run on 9mm all the way down to 22lr. Different pistols may require different pistons even tho they are the same caliber (H&K for example)