Agreed on the "specialized units" bit there Brad... I've always loathed the term "Snowtrooper", or "Sandtrooper"... The U.S. Army isn't formulating squads of troops specially trained to fight in the sand... It's impractical.
I think what you see with the "Sandtrooper" is that the term's fabricated in EU ignorance. Sandtroopers are merely Stormtroopers in their field gear. Shoulder pauldrons are used to denote a general idea of rank structure for civilians as well as the troops themselves during combat conditions (things in war get hectic, so seeing who is who can be important). The downfall of this though is the same as the downfall of all singling of Officers and higher-ups in history... They become targets. The British officers on horseback in the F&I or Revolution, the Officers targeted by snipers in WW2, WW1, Vietnam, Korea, etc... In the vain Empire though it definitely falls into place I think...
The "Snowtrooper" is to me the same idea really then... To me, they aren't Snowtroopers, but rather they're regular Stormtroopers in cold weather equipment. More flexible armor/uniforms for the terrain, heating gear, thermal stuff... It's like saying the US sent "Cold Weather Soldiers" to defend Bastogne instead of just throwing trench coats and Mackinaws on the 101st Airborne and others, and throwing them out in the field.
Now, that's not to say I don't believe in specialized groups for the Empire... I think they'd employ special forces units for certain, train them, etc., but what are the odds a Destroyer keeps a garrison of Troops on board for every environment? Not likely, even with the grandeur of the Empire and what they are capable of. Instead, the troops dress to match the environment...
Specialized units as I see them include:
-Scout Troopers (Not special forces per se, but trained separately of regular infantry and tasked with specific duties separate of the basic infantryman)
-"Airborne" units (requires a special training, but they'd still be merely a step above a basic infantry unit, and really it's feasible to assume all Stormtroopers get some "airborne" training and that "airborne" units don't even exist really)
The other group I'd consider CLOSE to Snowtroopers (and likely looking just like them) would be some kind of special Mountain/Arctic Trooper trained specifically for the environment... But again, do all Destroyers (Executor included) have a legion of these guys aboard? Highly unlikely. Instead they're likely trained and kept in reserve or immediately placed on planets the Empire holds that require their services. These troopers though would be additionally equipped for the perils of polar regions on planets like we see here, or high altitude mountains and things.
I don't see the Executor just happening to have a pile of these guys aboard ship though, much less a full group to storm a well manned rebel base... That just makes no sense. They're out hunting the rebels, they have no "plan" or assumed location to base who they need to bring along, so they're just going to have the troops they think they can apply to any situation.
Sandies though have always been a comical group to me. Their packs alone suggest to me a modular design built to be customized for the trooper's need. Special gear for goign into a forest, a swamp, a volcanic/lava planet, a jungle, a desert as we saw, a cityscape, ship board/capture operations, siege operations, etc., etc... They're your joe soldier with all the gear he needs for any planet he may be dropped on. For the deep cold, a new uniform is in his closet ready to roll.