Flours have many different textures and compositions, so are better for use in some baked goods than others.
Things like more or less gluten, raising agents etc that make them better suited to different applications.
e.e. plain four does not rise the same way as SR flour, so some recipies that have plain flour will also have bi carb or baking soda in them as well. - So you probably could use SR flour instead, its worht giving a go to experiment.

As fpr sugar i would never use castor sugar in place of icing sugar (or pure icing sugar - different again) they are entirely different if you get a small bag of each you will see what i mean.
Icing sugar is a fine powder compared to castor sugar, which is finer than sugar, but no where near icing sugar.

Castor sugar is good for recipies here you need to cream butter and sugar together, icing sugar for icings, pure icing sugar for royal icing.

You may need Niliac the domestic goddess for a proper rundown