As to whether there were more Myrkridia than Myrmidons:
I rather liked the "great white shark" analogy we had a while ago. The Myrks are (presumably) carnivorous top-level predators who don't understand agriculture (and don't eat their veggies anyway). If they have any kind of culture it is restricted to the maintenance of skull platforms (though that they might have a standard confuses this issue considerably).
The Myrmidons were human beings, originally (did anyone ever decide if Zerks and Myrms--and Firbys for that matter--were "human" or just very similar?). Though they probably spent most of their time primping and didn't care much for agriculture, they did have a considerably advanced society--at least compared to the Myrks.
Now, look at how many Great White Sharks there are in the waters off Australia and how many People live in Sydney. For some reason there are a lot more of the people. Why? Because the shark population is limited by the available, ambient food supply while the people are capable of producing their own food and therefore can breed enough to keep their numbers growing exponentially.
It therefore seems logical to me that there should be more Myrms--even if Balor only took the men of battle-faring age (I believe I saw a quote on that somewhere... awfully sexist, but who knows? Maybe the women were smarter and saw through Balor's offer). The Myrkridia may have been a "continent-spanning race" but if there were more than a few thousand of them they would have eaten the Mythworld clean in a generation or two. Top-level predators need to eat several times their body weight in dwarves every day to keep up their energy and support their hyper-evolved biology.
Anyway, that's the way it looks to me.
