First...Thank you for your service.

You asked about Spec Ops sheaths...
511 is too small for the Small Combat Master, but it does fit loosely...not my first choice by any means. I had Al (AzWelke) create Kydex for my 511, 711, and many others. Excellent work and quick turnaround.

711 is too big for the Small Combat Master, but fits into the large Combat Master. After sheathing, I'd make sure my handle strap was snapped before any activity.
That's where custom kydex shines, you get a firm hold on the knife without additional straps and things to mess with / worry about.

The Scrap Max knives are wicked thin slicers and are satin (shiny and reflective). Exchange is likely best way to get one in the shortest amount of time.

The 311 might make a nice fixed blade for belt carry.

And if you want another knife to add to the options the Swamp Rat RatManDu is an excellent all round contender in the 511-711 size range. Very comfortable in hand.

I have or can take photos of any of this stuff, just ask.