My Voodoo works fine in the topmost PCI slot (next to the video slot, farest from the mediator logic). I just added a PCI network card in the slot next to the mediator and the Voodoo still works.
I also run an X-Surf, HyperCOM 3+, FastATA, eFlash and Highway in the same board. No problems here.
IMHO, if it mechanically fits, it should work. You could put some plastic or paper between the cards to avoid short circuits.
Bye,
Thomas