ruben had fish pie with salmon and cod last week, and he loved it. i'm pretty sure it's fine!
why don't you take some of your normal dinner out before adding salt (ruben likes other seasonings, like cinnamon, nutmeg and even curry powder!! so i just omit the salt from his portion), and mash/puree it - i've done that for a few weeks now no probs. and i give him some whole pieces as finger foods as well at the same time.