sometimes when i place a variable in the wrong order i get errors about enumeration. for instance <SecondaryType> has to be right after improvement type, <CanSendTradeMissions> and <NoGalaticBazar> are also very strict about the order they get placed in. Might be you encountered something similar?
I have no clue about your modding limitations question.
are you trying to make a whole new starbase all together? That might not be possible. Making a new ring type should be I think, and if not you could either take say the economy trade modules and change those or perhaps some of the military starbase modules?
If you have checked nexus mods and there is nothing introducing new resources than it probably isn't possible. it was stated we couldn't add strategic resources and i don't think that has changed.
I really don't think we can change a lot about victory types outside of the following in GalCiv3GlobalDefs.xml
<!-- Tuning for the victory progress readouts -->
<!-- Will read 0% until we hit this value, eg at start of game. -->
It probably is not possible to make multi tile buildings. Even if you could somehow manage it the game doesn't seem to be set up for that or to know how to handle them properly. Neighbourbonus gets tricky if you take up multiple tiles, as would building level and trying to upgrade it. Get invaded and they destroy half the building? It may be possible to make a building type that gives a % bonus based on how many of that building there are, or maybe if it is next to it even? But i think that is as close as you could get.
So for summary new :
-Victory: probably no
-Answer in diplomacy: going to depend on what parameters it is checking or looking for, but there is the custom faction diplomacy editor so there should some tools and some stuff should be quite doable
-Trades: not sure what your intent was about this bit
-Resource: odds are no
-Starbase: not likely
-Movie: I don't know
-StrategicIcon: no idea
-Improvement size: no
-Starbase Modules: yes, but probably limiting to collecting or modifying existing values
-Alerts: same as trades
-Design: too vague, and no idea what you want to do
-AI change: unsure
-Map parameters: size likely yes, composition probably, spawning new resources ect likely no