We are speculating about a lot of "unknowns" here.
The reasons for Viking Age are very debatable. One of the theories actually suggests that there was a population boom.
If warfare was the reason for future vikings to have more children then that would be the case for other Germanic tribes as well thereby making claim that Germanic tribes had few children inaccurate.
Scarce resources never stopped people from breeding. Low population does not necessary mean fewer children, it can be do to fewer families to begin with or/and with high infant deaths.
I am not sure what to do with your information about Fins
You ought to acknowledge that the evolution process does not take hold in a period of perhaps 200 to 300 years. If the Russians, for example, are separated into two people, and forced to live on two different parts of the Earth, they will not develop to be genetically different from one another in period of a century or so. We are talking about tens of thousands of years of development overall, which separate the European people from the African (particularly those populations who have not seen much interactions between one another in the past tens of thousands of years). To understand how physical differences may have developed, one has to observe the general trend, rather than grasping to the exceptions.
I believe the Viking age represented a deviation from the norm, in regards to the Nordic way of life. It spawned as a result of an external threat, Christianity, and the expanding Christian empires. Previously, as the climate had grown tougher than normal, and as technological innovation allowed for increased mobility of people, but without having developed to the point of allowing the continued sustainment of excess population, Nordic people had abandoned Scandinavia, conquered lands in Southern Europe and settled there. The Goths, for example, were believed to originate from Scandinavia's harsh climate. The Visigoths eventually settled to Spain and the Ostrogoths eventually settled to Italy. The Vandals conquered lands in North Africa.
https://en.wikipedia.org/wiki/Goths#Origins
https://en.wikipedia.org/wiki/Gutasaga#Emigration_to_southern_Europe
The information about the Finns is meant to enlighten you on the primitive way of life in northern Europe, before farming, building houses, making settlements. It required extra-ordinary capacities from the individual, and a close attachment to nature, and exceptional understanding of it. It's a very different way of life from what, for example, the Mediterranean man would've been accustomed to.
There is no way that families of 7-8 children could've been raised under such conditions. That is the condition under which the Nordic man has operated, for the better part of the tens of thousands of years that he has been inhabiting Scandinavia, and other Northern climates. The Viking era, or the era of Germanic migration to the South, or this era, would be a blip on the radar by comparison.
