OP,
I'm an active duty Marine. I can verify that the posting of an MOS, what you may/may not use a knife for, etc are not violations of OpSec. To claim that the posting of your NG MOS aides the enemy is purely fictitious and makes you look foolish. I can understand not wanting to post private info, but no need to lie.
Now, onto knives. You say you are a POG that hasnt deployed, and you want a knife in case your weapon goes down. Well, my first recomendation is start learning some remedial action for weapon failures. A knife in AFG for self defense is gonna help you maybe .01% of the time. Learning how to tap/rack/bang will help you far more than a knife. If your budget is $100, look into an ESEE 4. It has a molle sheath, is near the size you desire, doesnt take up too much room on your load, and will handle all survival tasks you need. The reason why that knife will be better than some combat abortion is because 99.9% of the time, it will be used while you sit in the FOB to open care packages, unscrew battery compartments, or to scrape some carbon off of a bolt face.
I can tell you havent been in long, and thats ok. Seek out some mentoring from a senior NCO, let them guide you and help you make informed decisions.
Oh, and stay away from SW knives. They are a fine firearm MFG that decided to soil their name with their knife line.