Are proteins fungible? I thought different protein sources provided different amino acids.

In case of plasticizing it, it should be fungible in terms of use the cheapest and/or least healthy to make plastic-like single use goods.

That said, the question I have is given the nature of calcium (and its relation to limestone), I'm wondering if the milk protein here is especially useful to plasticizing because of its calcium content? A quick search does classify calcium caseinate as both a protein and technically a calcium salt.

No they are not but they can be combined to produce a "complete" amino acid profile.