Well, The Core Rule Book already has Humans as a core race
Elves and Half Orcs are in the very back of the core book
Drow/Dark Elves are in Alien archive
The Swarm pretty much ARE Tyrannids
The Necrons are Robot Zombies. So you COULD use all robots/constructs/Androids. For example, an Android Mystic could make a decent Necron Lord. OR you could create a new race from scratch.
The Tau are the only ones that you would simply have to create a new race from scratch.