Thanks for pointing this out. I adjusted the documentation accordingly. Even if you are not using all of them, people usually just go with the default 6 types for historic reasons, only extending it if more types are needed.
That said, it is in principle possible to go down to 2 types, you just have to remove the respective statement in src/io/parameters.c and adjust your initial conditions accordingly (in the header of the hdf5 initial conditions, the NumPart_ThisFile and NumPart_Total, NumPart_Total_HighWord and MassTable fields need to be of appropriate length). I was able to do this e.g. in the Noh_3d example. However, this only works smoothly for the hdf5 format, as far as I tested.