Let's say I want to make my mod only available for Terran for now, what is the best way to exclude Protoss and Zerg from the game?
Of course the best option would be to tweak the lobby drop-down, but in case this is impossible I'd be satisfied with just changing all player's races to Terran when the game starts. I'm not sure how to do that either though:D
Why do you say it is impossible? I think both your options you state are valid and common options. You can limit the lobby drop downs through the use of "Game Variants" which I'm sure there is a tutorial for in the link in my signature.
For simply disregarding the race dropdown in the lobby, I would hide it, using the "Player attributes" tab of Game Variants to set access to "hidden". Then use the trigger "Set Player Race" to set everyone to what you want.
I hope this was explained in a way that helps you, let me know if you want me to go further on part of this.
Interesting, I've never seen that before as I don't work with mods often.
I found a potential solution though:
In Data, open the tab for Edit Advanced Game Data -> Races. Pick a race there and you'll see a field called Flags+ which has a field called "Selectable". Try unchecking that and see if that prevents people from picking that race.