Granted, I'm no expert, but reading the CRL you must have the pauldron and ammo pouch if you're building a TD. Backpack and weapons are optional but encouraged. So the answer to your question is: Yes. You can build a sandy without backpack and still wear pauldron and shoulder-pouch. To state it accurately, if you build a TD, you MUST wear the pauldron and shoulder pouch.
You'll find as you do more studying that there are a lot of "odd" uniforms and armor configurations that appear on screen in the films. Even though they appear on screen though, that doesn't mean that the particular configuration is acceptable for 501st approval. The CRLs are always going to give you the guidance that you need to complete a uniform for basic approval (and higher levels where applicable). If you pay attention to the movies, you'll see TKs with no tube stripes, no mic tips, tape on their armor, missing pieces, etc but that doesn't make it acceptable.