Those 2 types of fish are very different so I would say you could add either first. Personally I'd add a blenny first and wait a week to see how he does. Also when you add the tangs try to add both at the same time and try to get the same size.
All this is prefaced by the fact that you need to wait till those nitrites are at 0 although 8 weeks of cycle seems pretty long so you may want to test again and see if you get a 0 reading. Are you running any macro algae in the sump or any GFO
to reduce the nitrates and phosphate?