Dunedin is one of the Commonwealth's "garden" worlds. If it were in the Inner Sphere, population would be 40-50 times as great. What I am thinking is this: only one or two billion+ populations, everything else in the 100s of millions (uncommon) or 10s of millions (common), or even smaller populations (VERY common).
So how many worlds/systems should the Confederation have? A dozen? Two dozen? Forty?
MA
As I understand it this is a SLDF colonization of the Anti-Spinward (something I've written a couple of times) region somewhere beyond the old Rim Worlds arm.
1. Well first off your capital world is likely to have the most people which will establish your ceiling. Twelve billion is the top I know of in the BT universe and that was Terra under the Star League so ain't no way you have a star empire of thousands. The mid billion (five or so) is a House Capital which of course controls hundreds of worlds (say 200 to 500) and I would say unlikely for your setting. Low billions I suppose is possible as we have examples of as Periphery capital worlds (Canopus and Taurus) but I would not go beyond one for this exercise. You are not the Magistracy or the Concordat who each I believe have one, two, perhaps even three of this third tier high population worlds. They exist far longer and closer to the InnerSphere than you. A Tier 4 call it, hundreds of millions, capital world along with a few others is more plausible to me. Say half a dozen (6-8 at most) with populations of 500 plus million denizens is more likely. Go with 6 if you do a low billions capital. If not maybe 9 with many millions as your core worlds.
2. What date are you guys trying to write to? And the era is?? You have a few choices of course. 3025 is always a good starting point as it is where BT began in the first place and I'd love to hear the story of why this bunch of noble do gooders didn't get involved in the events of the InnerSphere or maybe they did? Or is it 3050 and what does this True Corps of the Star League to do now with the Return of Kerensky to the InnerSphere in its shocking Clan form. The Jihad of 3067, do Blakist find them and involve the Southern Cross in the events of their Holy War???
3. What population and technology do you have to support your force? It looks like you are trying to maintain (gulp) a SLDF Corps! That may not be possible. Such a force garrisoned 50 or more worlds during the height of the Star League and was maintained be its vast industry. You are in the sticks here kids. Just saying you might not have the industry to support this strength or the need for it as your area to defend might be forty worlds at the most.
4. Hidden Empire. I get the feeling we aren't advertising for the InnerSphere to come vacation here. So what happens to explorers and agents of the Houses and ComStar when they come calling? Perhaps they come as comissars among the refugees can't have them report back can we..
Few thoughts on your populace.
Initial colonists. Your core supporters who bought with you will obviously form the stalwart center of your culture but do they get overwhelmed by the next three population influxes I suggest? Are there idealists or reactionaries who want to go their own way in the future?
Refugees fleeing the early Succession Wars is very possible and given your location you could have a ton of Lyran, Marik, and Rim World denizens especially if your pro actively setup to rescue them. Such influxes which you don't really control over could lead to major problems especially early in the 1st Succession War (2785 to 2821) given the scale of transportation available not to mention if your trying to stay secret or a hidden empire. The scale of this exodus from the InnerSphere is going to slow as jumpships are annihilated but could continue through the 2nd Succession War say 2850.
Were there people in the region before their arrival? An Uplift Program to gather people as a resource around the region to give them a better life could work but poses some problems. Do folks want your help in the first place? Some are likely looking to get away from an unfortunate crash or colonial disaster that the SL might have helped them with but are there other groups like American Indians (Nomads or Survivalists) say who have established the simple life and don't want to be relocated to a reservation or be incorporated into your grand new empire?
A Liberator Program is something I could see this noble SLDF survivor force going out and smashing bandit kings, freeing slaves, capturing pirates, and bringing justice to the dark corners of this region.
Given all this I'd say your should be given MA comments between 24-40 inhabited star systems. 24 or more (upper limit 30) should be millions of people settlements as I think the Southern Cross wants to keep a well centered populace with the rest being low population colony worlds but it all depends on the time frame folks.