Hey Oliver, if they’re all part of the same level it makes sense that they can be in the same .blend file, and unless you are going to re-paint the other assets you’ll need to use a new texture. I really wouldn’t worry about it too much. For texel density, try assigning a UV grid to the walls, and then try to eyeball which common texture size on the props (512, 1024, 2048, etc…) most closely matches, but it doesn’t need to be exact.
I’d love to see the result when you’re finished!