Darryl rightly pointed out that the 65 year filling up the steppe period was way too short and we either needed to adjust the numbers of Celts upwards or take a longer time to fill the area of the steppes. With the time compression between Garnia World and Earth, we can get a lot more settlers a lot earlier, Darryl had very early on suggested that the Helvetii get brought over after Caesar defeats them and forces them to leave Gaul for their homelands. I was initially kind of against that idea, but I have changed my mind; if even 1/2 of the surviving Helvetii make it to Garnia World that means 60,000 more settlers on top of my conservative 120,000 that had been collected from all Celtic nations, and these Helvetii are supplied with wagons and grain; and these Helvetii are showing up within a year of elapsed Garnia time of the original settlers.
On top of that, Caesar's naked imperialist aggression in Gaul ultimately is estimated to have dropped the population by 40% from 5 million to 3 million, with roughly 1 million of those being taken as slaves to Roman markets and the other 1 million killed. These are not Caesar's numbers, these are the numbers of modern historians and archaeologists. Not all of those dead would have been battle casualties, some would have just been the victims of the devastation of war; if we migrate a conservative 10% of those dead to Garnia, that's another 100,000 people coming in during that 7 year period on Earth which is a little over 2 years on Garnia. Presumably they aren't all coming at once, so it should not be too hard to assimilate a few hundred refugees with their goods they could carry into the well planned community of the original settlers at each small migration. This brings our total up to, during the initial migratory period, about 280,000 people, mainly Gauls at this point.
Now, I was thinking that the steppes would have to be full before the Celts would run into the Sidhe empire and decide to conquer them, but Darryl pointed out something important that I missed. Dun Math, the holy city of the Druids and near the site of migrations, is on the southern steppes, closer to the eastern end than the middle or the west, and people are going to spread out probably concentrically from Dun Math, along the paths of least resistance. That means they are going to discover the heartlands of the Sidhe Empire, AKA modern Garnia and it's rich green lands and want it soon, probably their explorer types will have found it within the first generation of getting there, but with the steppe so much easier to conquer and so full of game, and the Sidhe having their powerful Illusions and Charms, the first few generations probably don't stand much of a chance against the Sidhe.
But, there will be a great influx of refugees again during the Roman invasion of Britain, in 3 main waves starting in 43 AD. I can't seem to find any good source of pre-Roman population figures for Britain with a quick Google search, but it does say that 70,000 Britons died during Boudicca's revolt some 30 years later. I think that between 43 and 96 AD Earth time we should be able to pull out an additional 100,000 People; that's 34.3 to 52 years in Garnia after the initial migrations to make room for another 3 major migrations, of about 33,000 people each, assuming they're all roughly equal in size. The British language is pretty much the same as Gaulish, so there is no real linguistic barrier, I am thinking the original Celt-Iberians, Picts and Gaels and our possible Ligurians are starting to feel a little pressured.
But we still have a common cause, religion, culture and similar languages. 34 years in would mean that a second generation had already been born here and a lot of the original settlers, the adults anyway, were dying off; at 52 years in we'll be lucky to have any original settlers left and a third generation is starting to be born here. The Sidhe are also starting to lose their advantage in magic, Human magic may be raw and brutal, a little wild and prone to mistakes, but we don't live long enough to test it like they do. We've figured out how to counter their spells and whipped up some mighty offensive stuff of our own while we were hunting the disorganized Humanoids and other Monsters that live on the steppe.
Now I am going to do some math. The first 2 1/2 years bring in 280,000 people, they have a conservative population growth of 75%/20 year period. Some have asked if that was too high, I counter with the argument that the Celts have both a technological edge, with horses, the wheel, and iron weapons, not to mention agriculture, fishing and a host of other skills (including at least one form of literacy); but on top of that, most all of the Celts magic on Earth was dedicated to one of three things- either making them victorious in battle; or making them more fertile, their women, their land, their flocks and herds, abundance was the order of the day; or healing, which is going to keep more women from dying in child birth and more men and women from dying in battle, it will probably lower infant mortality too. So, 75%/20 year generation population growth it is.
490,000 after 20 years in Garnia World, 857,500 would be their natural population growth, but they are going to get an influx of 66,666 Britons in this time frame too, half of which will have the time to breed here too, bringing our total to 949166 at 40 years in Garnia. The next generation, 60 years in will see the last wave of Britons, but they'll be early enough to count for breeding, so our 60 year population will be 1,719,375. The Celts are one of the few societies in the world that were "top-heavy" on the warrior caste, and the steppe life is only going to make this worse, like the Huns or Mongols or Bulgars, pretty much every able bodied person CAN be mobilized for war, and they can easily do 10%; 171,937 would represent the nobles and their warbands alone, the rough steppe life means I think they could easily count on pulling in some pretty tough non-professional warriors too, say another 10% of the total population, mostly male in both cases, but not exclusively.
60 years in can be the time of Gwaryn the High King and the Sidhe War. I don't see the Sidhe going down that easy, and I don't see Gwaryn living to see their ultimate defeat, much like Genghis Khan didn't live to see the defeat of the Sung. But he will live long enough to lead his people off the steppes, and into the most fertile farmlands in the world, he will be remembered as a great warrior King, and I think that, in the end, elective Kingship will lose out to the line of Gwaryn; but not until after the Sidhe are defeated and the fallen Elves enslaved.
I also find it interesting, in a good way, that the time compression keeps us closer to the ancient world. Now we don't have to find excuses to make Ancient Romans a credible threat, everyone else is, more or less, on the same page technology wise, or far away and without the technology to come over and interact.