In your example "It's a nice C code snippet", the only modifier is the adjective "nice", and that modifies the noun "C code snippet". In turn, "C code snippet" is a compound noun made by combining the noun "C code" and the noun "snippet". In turn, "C code" is a compound noun made by combining the noun "C" and the noun "code".
That is, using catagory labels with left square brackets, we have the noun phrase "a nice C code snippet" with the structure
[NP [Det a] [Adj nice] [N [N [N C] [N code]] [N snippet]]]
Since usually compound words have most stress on the first element while phrases have more stress on the last element, the above structure for this noun phrase predicts that "nice" will have less stress than the following noun "C code snippet", while "C" has more stress than "code", and "C code" has more stress than "snippet". I believe this is correct.
The first parts of compound nouns are sometimes said to be adjectives, but although the interpretation of the compound may suggest this, I think it's wrong. Grammatically, in these examples at least, "C" and "C code" are nouns. You can't have compound nouns containing an adjective modifying a following noun; that would be a phrase rather than a word. Your example "! It's a C nice code snippet. [Incorrect]" is bad because the adjective "nice" cannot go inside the compound noun "C code".