You should really read the thread you are borrowing from.
The leathers you can embark with are related to the creatures you have available at embark. If you have a creature who's tanned hide will become LEATHER_4, then you will have it available. In MW, you get to embark with Tough Leather because of the pekyts, and rigid scaleplate from the drakes. You don't get to embark with rigid chitinplate because none of the embark creatures can have their hides directly tanned into it.
When I added some of the creatures that have the Mythskin and Deephide, they became available at embark.
Edit: I hope you've figured out by now that your setup will not be friendly with the Workflow GUI. Putting multiple items under a nameless creature means that to specify one of those materials, you will need to look through the entire list and locate the blank entry for the creature that you have it under.
Edit2: If you want some creatures to maintain their more valuable leather, then don't use the BODY_DETAIL_PLAN for their leather. Remove any leather that you are getting from a BODY_DETAIL_PLAN and then [USE_MATERIAL_TEMPLATE] in the creatures that you want to have more valuable leather. Not doing this will mean that you "leather" value x and "leather" value 2x. Remove the BDP leather and putting [USE_MATERIAL_TEMPLATE] will give you "leather" value x and "cow leather" value 2x.