While I would never plan such a short transfer time on two different tickets, you have them now. Chances are you will make that flight with a few minutes to spare, if you arrive on time. Arlanda is a small airport...even if they tell you you'll depart from another terminal. Unless I missed a terminal, the terminals are connected. Check the airport plan to make sure there's not a separate terminal hiding from me and know where you are headed. Immigration was fairly quick at Arlanda, and the airport was not particularly crowded (I have a Logan Airport frame of mind). This is not a large airport. It strikes me that your Norwegian ticket would not have been particularly expensive...I'd never risk this with the expensive ticket being the second flight. However, your ticket is purchased so I'd take the chance, but I'd rather get stuck in Stockholm than most other places... ;) And I'm sure there is another flight that day considering your early arrival time, at least one, or there is always the train.
PS. You will leave the secure area, but security was essentially nothing when I traveled there in 2015...could have changed, but you are now traveling on an internal European flight. I've not found much security on those...IMO. However, last such flight was in the spring of this year so could have changed...
