How do I get all the textures onto one tga file like all the original game meshes? I saw something about it in the showcase thread but I couldnt quite understand

what do you exatly mean?
so you want to know how to make UVs like in the shipped textures?
you dont have to put everything on one texture the jka models use about 5 textures

Well there's no particular reason to, but if you want, you'll have to put everything together in photoshop/GIMP, then re-UV the model.

I'm pretty sure he's just asking... how do you UV? (Which, by the way, is something I've never understood either, so... )

yeah, I've never understood UVing. Just [edited] cubic/spherical for me!

Simple merge together the objects into one. Is that what you mean?

http://en.wikipedia.org/wiki/UV_mapping

just play with it in XSI and youll learn by doing, its easier than someone trying to break it down for you. its best to have your model as a single mesh using a single UV set and single UV texture since SWBF2 uses fairly low poly models. having a bunch of separate meshes per model cripples game performance especially if the model is in constant use i.e. a common soldier.