Well, I usually change these things as they arise.
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Sysgen is one pet hate I have with Traveller, and one area that, thanks to its interconnected complexity, I've never got round to houseruling. One day...
I firmly believe that a set of unvetted UWPs is just a draft and that any company that publishes one is producing unfinished work.
My house rules:
1) Go through the UWPs. Put a check mark against any that doesn't puzzle you.
2) Go over the rest one by one. For each one, spend 10 minutes coming up with an explanation for the oddity. If you come up with something, note it down and put a check mark against it.
4) If you didn't come up with anything, give it a "saving throw against weirdness" from 1 to 5, depending on how weird you think it is. If it fails the throw, make the smallest change to the UWP that you think makes it work (Or the change that makes it most useful for RPG purposes) and put a check mark against it.
5) If it makes the saving throw, spend 10 minutes trying to come up with an explanation that isn't too implausible. Accept low-order probabilities, unlikely coincidences, and anything else that works for you. If you come up with something, note it down and put a check mark against it.
6) If you didn't come up with anything, give it a second saving throw. If it makes it, spend 10 minute coming up with an explanation involving once-in-the-lifetime-of-the-galaxy weirdness or the Ancients or really unlikely coincidences. If yuo come up with something, note it down and put a check mark against it.
7) If you still can't come up with anything that works, change the UWP already!
You simply can't have a Class A zero pop world, certainly not at TL-0.
Pop-0 is under ten people. How are they going to build starships? Even the Morgan car company has more than 10 employees, and they only produce ground cars.
The usual handwave is that the starport is run by transients. I don't like that one for a number of reasons, the two biggest ones are that a world with transients would still have the trade effect of the same number of permanents and that you need to come up with an explanation why someone is willing to pay the added expenses of building the shipyard, keeping it manned, and importing every subcomponent and every bit of other equipment needed to keep the workforce effective.
(Not to mention the political oddity of anyone accepting a small handful of people as a sovereign population).
Hans