Can I re-download the default keyboard (iOS 8) after deleting it? I'm planning on making SwiftKey my default keyboard and deleting the "stock/default keyboard" provided by Apple. But first, I want to make sure I'll be able to get that keyboard back if I need to. 

Will I be able to get it back after deleting, and if so, how?

iPhone 6 Plus (iOS 8.1)
Yes. You can get back the keyboard without having to redownload the keyboard, It's never gone. You cannot delete the stock keyboard, you are only removing it from the default set of keyboards. The native keyboard can always be added from Keyboards &gt; Add a new keyboard &gt; English whenever you want.