Hi IeM,
I could solve my need with te help of your tip and using filter and display params. In my case I only needed to get the current stock quantity available. I didn't need to update it.
The awkward thing is the fact that calling the products end point always have a 0 value for the quantity attribute but calling the stock_availables end point returns the correct quantity. This just enforces my last message, PS dev team implemented WS as an afterthough and they don't support it at the same level as the other components and features of PS and that's pretty sad.