There isn't a download limit on ebooks or movies you buy.
The "6" limit you are referring to is having the same purchased ebook downloaded to 6 devices at once. This is to prevent you from buying a book and letting 500 of your closest friends download it together.
6 devices can have the book....if a 7th device wants the book, it has to be removed from one of the others first.
However, you could download the book, delete it from your Kindle and redownload it as many times as you want.
I believe the same goes for movies but I don't know if its a 6 device limit or less. 6 Fires registered to one account would be a bunch!