Yes, they can stack. You have to deal with Armor Encumbrance if you lack sufficient strength for the +3 armor. You can also stack the Synergist Long Coat with the Steampunk armor as well, if you were to go that route.
What kills me, though, is how Sleeping Tiger suddenly has Ruthenium Polymers. Up until Run & Gun it was always just elctrochromic modification (simple color change, not active camouflage). I don't know that it will ever be errata'd, but I house rule it more in line with the earlier edition descriptions, and, as such, drop the cost by ¥9,000 to compensate for the difference.