You can use Wings3D to make your models, but you will need to export it to a program that can export to xsi. SoftImage (SoftImage Mod Tool - free version) or Blender are the only 2 programs, that I know of, that can nativelly Export to .xsi. Wings3D and other programs may have plugins that allow them to, I'm not sure. But if not then you will definitly need either SoftImage or Blender.
Gimp 2.6 (free) is a great program for doing textures, But there are numerous others like Photoshop, Zbrush, 3Dpaintbrush to name but a few, if you got the cash to spend. Your finished result will need to be in dds. format so a plugin will also be needed for Gimp & Photoshop. I'm not sure wether the other progams need a plugin as I have not used them.
On a side note: I have finished the pre-liminary structure of a mod that does add extra capitals (1 for Each race). Whilst they are not what you would call Titan class, they are more powerful, and my aim was to make them fit in to the SoaSE game how it is now. It still needs a bit of touching up texture wise and I havn't added a theme load up screen or anything like that to it yet. You may find it helpful for your mod and you are welcome to use it / parts of it to make yours. So is anybody else for that matter. It is only for Diplomacy 1.011 though, so that is needed
It can be downloaded from here; http://www.mediafire.com/file/uzjonihjnnr/Eclipse, Hidden Alliance's.zip
Another thing I almost forgot to mention was "Google Sketchup" you can use for making models. It does'nt export as xsi. but does export to obj. which SoftImage and I think Blender can import. Google3D warehouse has some spaceship models free to use, if you wanted to add those also.