I trained up two of my favorite soldiers (Mark and the blond-haired kid) in Major Sniper and Sneaking along with Major Athletics and other skills. I put them in the Light Armor that you cannot see their face and was wondering why they cannot sneak in this armor? Which armors can they use to sneak around in? I've got them both with Customized Rifles and Sniper Rifles with silencers so they can do hit-and-run missions (they work real well so far) but I had just noticed they couldn't sneak in this armor.. Is there a reason why? I thought they would be able to sneak in ANY light armor.

To unlock the prone and kneeling stances aswell as the crawling movement type you need minor suit wearing. The stealth training simply adds a bonus factor to it's stealth. IIRC, the "lower" stances and slower movement types increse stealth against some detection types. There's no actual "sneak mode" that you can set your units to.

I do have Minor Suit Wearing but when I switched to that Space-Ninja-Like suit It seems that I lost it, but I can still crouch and run.. I like the suits as they look much more stealthy, but I just can't sneak or whatever..

Only light suits allow crawling and going prone, the "Ninja" suits are considered to be medium size. Of course, mods can change this.

Yeah I didnt realize it was medium armor at first because it was in the light armor section in the squad screen. I may try and find a mod to make it light armor though. Thanks.