You can successfully run AppleTalk and TCP/IP protocols over the same network. More specifically, A/UX machines running TCP/IP have coexisted peacefully with Mac OS machines on the same Ethernet cable. Note that "coexist" does not mean that the machines can communicate with each other. Coexistence means the ability to use both protocol stacks on the same network without packet corruption and data loss.
AppleTalk has been designed to be compatible in multiprotocol environments. MacTCP (with classic networking) or TCP/IP (with Open Transport), and AppleTalk also coexist peacefully on the same network.
The area in which you might see some difficulty is routers. Not all routers route both AppleTalk and TCP/IP packets. Therefore, if you need both packet types to be transmitted, you need to ensure that your routers or bridges are properly configured for handling all of your packet types.
Protocol incompatibilities are unlikely to be caused by the actual card used. An EtherTalk card and a Kinetics FastPath card should yield similar results in terms of being able to run multiple protocols over the same physical media.