The Creeks and the Iroquois didn't exist as such at the time the Norse were colonizing. The Creeks arose out of the survivors of the Southern Mississippian culture after it was destroyed by the epidemics traveling in the wake of Hernando de Soto's expedition in the early 16th Century. The Iroquois Confederacy also formed, for different reasons, in the mid-16th Century. And you ignore the disruption that the huge epidemics caused by sustained contact with Europeans....smallpox and Black Death, to name two major ones...would certainly have caused. It will take the natives some time to recover from these, and in the interim, the Norse could greatly expand at the expense of the Skraelings. But your main point is good. The introduction of the European livestock, grain, and ironworking techniques would cause far-reaching changes, and make some of the native tribes (once they have recovered from the epidemics) formidable opponents. Most of the eastern tribes are probably toast, as they will not recover sufficiently to absorb the technology and food items into their cultures before the Norse move in and take over. But the tribes west of the Mississippi may be a very different story. The idea of a "united native American army," however, is rather far fetched. They were never able to unite in the OTL. The same tribal hatreds are going to be there. I just don't see most of them uniting in this TL, either.