There is a certain minimum size the total buffer must be to operate on iOS. By default this value is 1024 samples. You can reduce this by using native AudioSession properties, but for performance reasons you should probably only go as 256 or 512 with 4 buffers.
You can read about latency for iOS in the new Latency section of the iOS CHM docs for FMOD 1.05.00, in the basic information section.