Hello, I use two different slabs for my projects, I use one that will be everything but the finishing, and other on top of that for the finishing, I work with an interior designer that is very picky and sometimes I have to do that every tile is a slab, in order to play with them and make the pattern he wants, and take measurements. But the simple way if you just want to have different finishes by room, will be, either doing a slab per room, or doing a big slab for the whole floor without finishing, and then adding a thin slab (the thickness of the finishing material) on top of it, and even putting this slabs in a different layer to have more control over floor finishing.
For the wall I found an interesting object that I attach here, is like a face that you will put into the wall with no thickness, and you can add a material to this, and you can resize it. I found it to be very handy, but honestly I don't remember where I found it, I thought it was in archiradar, but can't find it there. But don't worry is free. The only thing I see with this object is that it won't give you the real thickness of the finishing, so you have to add it to the wall composite.
Other way will be using the beam tool or the same wall tool to create a thin layer (again, thickness can be the thickness of material) on the wall, the same concept as the floor, just make sure the material you are using has a high intersection priority.
