COT320 and Arches Platine are both made by the same manufacturer. COT has greater texture than Platine. YOu will definitly need to treat socorro if you want a smooth print. It has a basic sizing that needs to be neutralized, when I treat it with hydrochloric acid, I can see the carbon dioxide bubbles rise from the paper. I imagine they are using some carbonate salt for archival properties. Given that this was supposed to be a paper specially made for pt printing I cant think of any reason why they do such a dumb thing.
Cranes Platinotype needs no pre treatment, and I have no tried Cranes Kid finish so I cant help you there.
Good luck.