Re 1: not necessarily. The drop from the little flanges which stop the box falling through the plate to the point where the frosted screen is just above the neg holder varies from enlarger to enlarger; it may be OK but may mean the frosted screen is sitting on the neg holder, or it may be too separated from it for even illumination. Certainly varies greatly; I have boxes for Focomat !!c and Durst DA 900, but 203 and 504 may be the same.
Richard
With the speed of modern papers, the 35mm box is not really needed. As the previous poster commented the light mixing boxes vary according to the mounting plate used to secure the 500 head to any given enlarger.