If I am not worried about file size I just do a single 3ds save and change the nurbs at that time. But if its ahuge file and I will often convert to meshes as I model. Just make sure you do a Nurb backup copy. Either in the program or during export the converting to mesh is the same. I prefer to do a quick preview before I export. A bit longer a less faith in my abilities. As far as scale is concern. Everything seems to come in at a defult size. I haven't had problem with mixed scale sizes but i haven't mixed Rhino files yet. Meaning if I have to bring something in or make a change its been from the origional rhino file and exported as it own 3ds format