IPad – cheapest iPad necessary for running Swift Playgrounds application

I think it makes sense to go for the iPad mini. This is because you can simultaneously test both iPhone AND iPad apps. The iPad mini will run iPhone-only apps in a reduced-screen screen mode, and will run iPad apps as normal. An iPod touch will only run iPhone-specific apps.

For the Kindle system, if one looks carefully under the product description such as you would find under this list, http://www.amazon.com/Kindle-Newspapers/b?ie=UTF8&node=165389011, one can see a link for "Available on these devices". This link will tell you if an Amazon subscription for that newspaper can be read on "Free Kindle Reading Apps", or exclusively on "Kindle e-readers".

In order to have the freedom to read on multiple devices (which seems to be the intent of your question, but forgive if I assume too much), you'd want to choose a newspaper available on "Free Kindle Reading Apps".