Can someone give me a(n) outline of what files or 3-d image files etc. are compatible with XSI?

any image file that photoshop or anything like that is compatible, softimage even has its own file for 2d images called .pic. the 3d formats are .3ds, .fbx, .fx, .xsi, .obj, .scn, .iges, .igs, .dsc, .eps, .bvh, .amc, .asf, .x, .vmf, .smd and XSI 6 has full compatibility with 3ds max and maya so pretty much any autodesk format as well. also if you know some scripting and have the XSI SDK you can write your own importers and exporters.

Thank you for that summarize, Mini'

I think the question has been answered