When I try to install the 3.1C patch, "ta1x-31c.exe", I get this weird message :

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Error
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This program has run into an error while attempting to patch your version of Total Annihilation. The most likely reason you received this error message is because you are trying to patch a version of TA that is not compatible with this version of the Patch program. Try downloading another version of the patch that works with your version of TA.
---------------------------
OK
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I simply wanna add the additional (official) units found rehosted here :

http://www.shonner.com/ta/units.htm

As well as the enemy galactic gates. However, in order to do that, im told to install the 3.1c Patch.

I tried uninstalling Total Annihilation, and reinstalling it, but only the main game (and not the expansion packs), but I still got the same error message. Anybody have any advice for me ?

Im running 32bit Windows 7, on a lenovo x60 laptop.

GOG version is already 3.1.

I had the same problem as OP while trying to install patches.
The problem seems to be: if the GOG version is already 3.1, why don't subsequent patches work that are designed for 3.1? Is it not possible to patch the GOG version?

There are no more official patches past 3.1

Thanks for the quick reply!
I'm running TA on a Mac using Crossover.
The 3.1 patch should support 500 units but I'm stuck at 250 which is why I wanted to try a unit patch, and was also interested in the unofficial but popular 3.9.01 patch that's in the works.

Neither of these seem to work with running TA on Crossover in this method.

If you have any ideas or insight, I'd be grateful!

You should have the following in totala.ini in the game directory:

[Preferences]
UnitLimit = 500;