Modified tech trees go in Mod Folder\Data\TechTrees\.  You don't need to include unmodified files as the game will read them from the install folder. 

You can create a new technology by modifying the XML files directly - just clone an existing entry and it'll show up when you next open that file in the tech tree editor.  You can then modify it so that it appears at the right place in the tech tree, and has the right branch/category/AI value (this affects the AI's decision about which tech to research next).  it should then show up when you load the mod, if you have picked a race with that tech tree.

Unless it has changed since DL, files in the ModFolder\Data folder will completely replace the original files in the Data\English folder, while files placed in a subfolder (i.e. \ModFolder\Data\TechTree) will append (add to) to the original file (or to the replaced file in ModFolder\Data).

That means, if you put a full tech tree file in the ModFolder\Data\TechTree folder you will probably end up with a lot of duplicate techs.

Thanks for the info guys.

In the case of making new techs, does the same hold true for improvements and ship parts as long as there's some kind of model attached to them?

Also...I'd personally kind of prefer to use the tech editor because I THOUGHT it would be easier...but it doesn't really seem to be able to do what I want.  Honestly, I don't really know how to use the thing.

Yes.  If it's not in their tech tree, they can't research it.  However, unless you play with tech trading off, they can still obtain the tech (and the weapons) unless you specifically forbid trading that tech.

You can make new improvements and components. using the respective editor.  You'll want to edit a copy of PlanetImprovements.xml and GC2Types.xml respectively to do that.

It takes a bit of practise to get the hang of them, and a bit of knowledge too.  One useful tip I can pass along (that I learned today) is that some ship components such as Armor Plating and Superior Duranthium show up in game even if you delete them from your modded components file.  But you can give them a tech pre-requisite of ImpossibleTech, in other words a tech which doesn't exist in any tech tree.  That means they will never show up.

No, not possible.

Something just occurred to me...would it be possible to create a minor race that starts the game with three (invisible or nonexistent) starbases in every sector, but nothing else?  No worlds to take away from them?  Or perhaps some other way whereby they aren't participating other than to take up space?

I doubt it, but I thought I'd ask.  I'd really like to keep everyone to one starbase per sector if possible by any means.

(beam) Laser    -->    Plasma Thrower    -->    Ion Beam    -->    Disruptor    -->    Spatial Beam

(gun)  Adapted Ballistics  -->  Railgun  -->  Coilgun  -->  Particle Stream  -->  Singularity Cannon

(missile) Adapted Explosive --> Directed Nuke --> Anti-matter --> Disruption --> Spatial Warhead

That's an example of the kind of pattern I'm wanting to implement, and throughout more than just weapons.  It's another aspect of an overall more realistic rebalancing I'd like to accomplish.

Engines especially are another area I'd like to redo big time.

It doesn't seem like it'll be particularly difficult, just time-consuming and tedious.  Ah well.