What's the minimum subfloor thickness required for hardwood flooring?
Agree with diceless that a floating engineered wooden floor is more normal over a concrete slab. Thermal movement in fully fixed wooden floors causes problematical expansion/contraction more often than not.

Even using stable wood, the humidity in the room will change season to season, as will the temperature of the underlying/surrounding structure, all can affect the appearance of the wooden floor. 

Was there a specific design/idea you had in specifying the ply subfloor?