[Loopy]

Yes, if you really, REALLY want to make it harder for yourself and be forced into something like 8 different units, you don't have to use Vikings. That's still not a good situation though. Armored Princess gave you dozens and dozens of options on the first islands other than just human units, it was much better.

Because of Skald's higher leadership (I wish I knew by HOW much more), they might become the "defacto" master of certain builds.

Thankfully the Viking/Warrior's berseker and bonus to crit damage bonus is INSANE and probably dwarves any leadership bonus the Skald/Paladin gets. I don't know for sure though.

It's a 1/.75/.50 relation between Skald/Viking/Soothsayer based on pure levelup bonuses. Other bonuses are either % based (which won't change the ratios at all) or additive bonuses (which will improve the viking and soothsayer's ratio vs skald). At low levels the additive bonuses of the strategy medal and Valkyrie will greatly impact this distribution, expect it to be closer to a 1/.80/.60 ratio (this is based off my soothsayer's additive bonuses on my save right before the spider boss, which is about the point where they will have the greatest effect). So you can assume that Skalds have between 25% and 33% more units than Vikings at most times.

Yes on paper, but consider att/def and resistances ratio of you and targeted enemy unit.
Expl : 1 black dragon and 315 thorn warriors attacking knight stack. BD will make 110-130 fire dmg with 70att so to knight with 27 def and 30% fire res it goes to average 120*(1+(70-27)*3,33%[this is increase of 133% for att-def])*0,7[this is 30% fire resistance]=204 dmg + possible burning effect
On other side 315 thorns 4 att, 1-3 physical dmg will do to same target 315*2(average dmg)*(1+(4-27)*3,33%[this is reduction for lower att then knight def])*0,7[this is reduction for 30% physical dmg]=280 dmg.

So thorn do make more dmg, but considering if knight stack retaliate they will kill some thorns, but hardly BD and next turn BD make same dmg, while thorns make less caused to loss on retaliation. And so on... Crit% is also factor here, but also is fact if you have 4999 leadership you can have 625 thorns, but still only 1 BD and then thorn will make much bigger difference in dmg uotput. Pure dmg/leadership ratio isnt only thing to consider in planing your army.

AFAIK isn't the scaling on att/def linear, and so the effective damage ratio is always the same unless resistances come into play or the enemy defence is more than +-60 your units?

Should really include crits in your calculations. Crits are huge.

Also important to consider is equipment and spells. Consider that Bless will improve non-crit damage by 50% on Thorns but only 8% on the Dragons. Crits improve damage by 300% on Thorns but only 62.5% on Dragons. Equipment that adds +1 damage will add 33% damage to Thorns but less than 1% damage on Dragons.