The rare garnishes are tiered, and require a minimum level of skill to harvest; for dragons that means a certain version of Dragon Ingenuity. You should get a message that tells you which version you need, ie Harvesting this resource requires Dragon Ingenuity (6) or something to that effect.
Kala, as far as the after creation garnish system, that is one of the core features of the Revamp. It is the reason you see two level "restrictions" on some of the foods. The second one is the upper limit of how much boost you can get from garnishing. Throwing a T6 garnish on a T1 dish will only boost you up to a T2 strength effect.
It was originally going to be much more involved, allowing you to add in almost any "logical" processed food, but was deemed far to overwhelming to implement. I don't see any downside to the system other than the seasoned foods won't stack.
Drev