With taninim it is so that their sorcerers can eat the materials and then specific scales change forms to represent those materials and thus the materials are no longer needed for casting. Now my question is what is if they take on humanoid form? Should they also have a representation of these materials visible there (like in the forms of tattoos)?
Following shape shifting rules for Druids, they retain their access to what they need to cast, especially if they take the feat Natural Spell. If subject to an Alter Self spell a character retains their creature type and can still use all their amenities, this is more of an appearance change. Contrary to these Polymorph results in everything you are carrying blending into your new form unless it would appropriately transition (like human to gnome, you would still be carrying your backpack and gear, but human to wolf everythign would be blended into yoru form) and deprives you of supernatural abilities, which using these scales would fall under. Look also at the answers for What abilities remain when using Draconic Exemplar's shapechange ability? This question at least in part is answered there.
If there is nothing that clearly defines the nature of the shape shifting, change form or Polymorph involved you will need this answered by your DM, or if you are the DM make a ruling.