Fish cooks very fast. You can lay it over the top of the potatoes for the last 5 minutes cooking. Or it might be better to microwave it, there won't be much difference at all in the cooking time and then it will be easy to check for bones as you flake it. You can see when it is cooked, it loses that translucency and becomes opaque. Make sure it is cooked all the way through and do check for bones.
Any white fish will do nicely, the supermarket Value packs are ideal - or Basic or whatever they are called depending on where you shop.
Before you drain the potatoes, scoop out a cup of the potato cooking water and set it aside - if you just mash the cooked potato and add the flaked fish then it will be pretty dry otherwise, you can add a bit of the cooking water to give a nicer consistency. No salt in this, of course.
With regard to ratio, I'd say half and half - as if you were putting together a plate of food for a person, same sort of idea as that really.
And yes you can store it for a day or two, let it cool then keep it in the fridge, covered of course.
