Nope - that's not a dumb question. It's quite a good question!

Potato starch and potato flour are 2 different ingredients. It doesn't help matters when they label a product "Potato Starch Flour" Which is it...starch or flour?? It's really starch. They are using flour in a generic sense.

Flour is the result of grinding up a food item (potato, corn, wheat , etc)

Starch is what is extracted from a food item. It's also much finer in it's consistancy. Used a lot for thicken sauces. It's much lighter than flour.

Potato starch and potato flour are not really interchangable.

I don't think I'd use instant mashed taters for potato starch - I suspect it's much too heavy depending on the finished product. Although I'd have to look at what goes into instant taters to know - we rarely use them.

Tapioca Starch and Tapioca Flour are the same things.

Potato Flour: http://www.ener-g.com/store/detail.a...&id=109&cat=10
Potato Starch: http://www.ener-g.com/store/detail.a...&id=111&cat=10

This is kind of cool - lots of info on differnet starches:
http://www.foodsubs.com/ThickenStarch.html

Some basic guidelines on what to use when (look towards the bottom of the page): http://www.csaceliacs.org/recipes.php