As I said earlier, FastEthernet.device needs ram from Voodoo. So yes, Voodoo must be connected before the networkcard can operate.
1) Not all cards need Voodoo. Like I said the 10Mbps network card doesn't, but 10/100 does. They need some amount of memory from Voodoo. In my experience Voodoo doesn't have to be in first slot, but as heard, for some setups it works better in it than on other slots.
2&3) Read the documentation provided with the Mediator drivers.