One note of encouragement...
The Official Traveller Universe(OTU), of which the Spinward Marches is just a section, was a follow-on develoment to Traveller back in '77.
The design of the game, which I feel is brought forward well in this release, allows for pretty much any GM designed answer so long as you remember one thing...
When gaming in The Third Imperium, what happens "on-world"(or down-well or dirtside) means nothing so long as the taxes get paid. As a result, safe and abundant commerce is the key to happyness. So if you look at a world and there is not a pre-generated answer ready to hand, make one up! Just make sure you do not break anything you already have in play or supporting your campaign. So there is no write up for Somem...
A good rule of thumb is to use GM management of the player's interstellar movement as a check to protecting your need for data. If the players have a Jump-1 or 2 ship(Detatched Scout / Couriers, Free and Far Traders are by far the most common in The Imperium) then look at their Jump-4 map(those worlds they can reach in two or three jumps. Read out the UWP's for those worlds t get a basic picture of the place... Then look for real world examples...
Ex: Dry, no trade code, lots of people, restrictive government... Chinese cities on the edge of the Gobi!
Ex: Water world with a democratic government... Iceland!!
In that last example, a Balkanized Water World... Hop on the web and read a bit on Iceland and break the "world up" by the Cities and Towns which become the basis for your nations....
So long as you develop a reason for trade...or denial of trade, you can flesh the universe out.
As for Denial of trade...
Take the Canon World of Porozlo(Rhylanor Subsector).
This Balkanized world has become the Encyclopedia Galactica poster child for Global War. Some of her nations trade with the Imperium and some refuse. Some secretly trade with factions from the stars while publically eschewing any off-world contact! This is not only a really interestnig place but...because of the refusals to trade in the open or at all, it becomes the place you GM's can provide a LOT of adventure to your characters in!
Marc