Disguise is a stealth gear perk that slows down how fast enemies detect you. It does not change soldier suspicion levels, so you still need to stay out of restricted areas.
When a soldier starts noticing you, his detection meter fills more slowly. You get more time to duck behind cover or take him down quietly.
Yes for stealth runs through bases. It is less useful once fights are loud, since detection no longer matters.
Digital Camo handles cameras and turrets, and Light-Footed cuts the noise you make while moving.