I haven't looked in-depth at what's possible, but considering the choices of home system available in custom race creation I'd say it seems like a definite possibility to add another one in there, and then make a new, additional tech tree with the colonization changes you mention. Doing a full tech tree seems kind of a hassle though.

If someone could do it (and as a traidoff make some penalties for this civilization in other colonization options), I would be really happy!

I don't want to push it thought if it's time consuming or anything. As I said - I have no idea about modding, I can only make graphical stuff myself so for now I just have my "aquatic" civilization portrait and background and I pretend like everything else is included

If you want penalties it shouldn't be too hard to do that, just change the default values for this civilization like maybe replace the standard production with penalties and replace the specific aquatic technology with technology to deal with normal worlds. It would make it very difficult unless you spawned extra extreme worlds but that would have a poor effect on all other AI as they need those worlds. 

I can't help on the modding front, I'm in the same boat as you I'm afraid.

But something you said caught my attention - you can make your own race portraits and backgrounds - how do you do that and is it easy to learn?

When I first started the game I immediately went to creating a new race to see what can I do with it. And I saw that you can choose portraits and backgrounds for your race and that there were pictures that were not used for main races. That's when I thought that probably I could import something of my own. And I did.

I found game folder and quicly made two pictures using Naija art from Aquaria (drawing completely new art would be too much, and I'm the lazy-ass).

Here's the result:

http://imgur.com/a/HT3i9

The edges a bit rough, but I literally spent like 2 minutes on this and I didn't want to get out my pen tablet

So, how to do it...

You can find in-game portraits in C:\Steam\steamapps\common\Galactic Civilizations III\Gfx\Factions

You'll need a foreground of your civilization and the portrait of a leader. And it isn't hard to figure out file names based on what we have in these folders. For example my custom files are Naija_FG.png and Naija_Small.png

After that I restarted my game and selected custom images.

I like your idea, hope it get fleshed out. Also thought your creature pictures were cute.

I think what you're refering to is called "drawing" Or one could always use photoshop to put together something from different pieces. Depending on what your goal is.

As I said - I made those pics in 2 minutes just for the sake of experiment. If I spend some time I will be able to draw my own, but I don't see the point unless it's for a fully fleshed-out mod.

Quoting krbjms,

reply 11

I like your idea, hope it get fleshed out. Also thought your creature pictures were cute.

Thanks! But to clarify, as i've mentioned earlier, Naija is not my character - she's from the game called Aquaria. I just used her image and some references to Aquaria in race's description.

I have my own fan-art of this character though.

I honestly doubt that. If Stardock wanted to make as you said "unusual" civilizations that would inhabit different kinds of worlds, they would already.

Endless Space did it. They had races that prefered unique environment and colonized different types of worlds with different success. Some liked it hot, some liked it watery, others prefered toxic atmosphere etc.