Vivaldi feet account for 18mm of the total height of the box. So the Rossini +clock stacked on top of each other they would be 153mm tall if you are thinking of using them minus feet to save height.Or separated you need 125 mm for the DAC ( 107mm without feet) and 64 mm ( 46mm without feet) for the clock.
The dCS manuals say on positioning " For best sound quality the units in the …range are designed to be mounted on separate shelves".
If I was unable to place them on separate shelves I would want to use some kind of interface ( e.g. pucks) between them. However I also think that most people on this forum appreciate that in most cases feet are necessary though some are trying aftermarket alternatives to the feet supplied. However I now appreciate that the spike/anti-resonant disc arrangement on your rack appear as if they may be used in substitution which I guess is what you are thinking of.
So you are OK with the 285mm available ( if that refers to height) but 100mm total for two additional levels to contain the two Rossini chassis is not practical.