Textures are controlled by scripts as are each and every element which makes up FPSC.
Look in the segment folders. You will see small icon and script file for each (building) segment. Open a script file in notepad and you will see how the textures are allocated.
The walls are easy enough to re-texture. For floors/ceilings look at my post "Segment Floors in "Feature Creep".
Make copies of any script or other files you want to alter and save under another name - dont change the default scripts.
Reading the Manual or FAQ here may help answer further questions.