THE GALACTIC SENATE

I thought there were 4 kinds of sandtrooper, with inference to Hasbro's toys. Given the fact there are 4 pauldrons, Orange, White, Grey and Black.

However, I realised I may be wrong. Some folks at rebelscum.com forums argued that grey pauldrons sandtrooper doesn't exist, and is a mere Hasbro's creation, because Hasbro thought there were grey pauldrons, which in fact, are reflection of white pauldron sandtroopers riding on dewbacks.
So there should be 3 types of pauldron.

3 or 4?!?

and do sandtroopers carry a canister behind their belts (reference to stormtroopers) given that their backpacks blocked the positioning of the canisters?

Can someone pro enough to enlighten me?

The OS and Visual Dictionary states only 3 coloured designations:

• Orange - Squad Leader
  White - Sergeant
  Black - Enlisted

I'm inclined to the opinion that the grey pauldron is either EU/fan-based material even though (or esp since) there's a mention in this FFURG article here. The ones in the movie probably are battle-damaged versions of the white pauldron.