At the moment, EOS_Omni::poly_gamma and EOS_Omni::poly_gamma_ini are two parameters for the adiabatic exponent, one used only for initial data, the other for evolutions. Both default to 2.0. This means that if someone, unaware of poly_gamma_ini, changes only poly_gamma the result is usually catastrophic (wrong results or c2p failures). I myself didn't notice today and had to debug my way down to find my mistake.
I propose to make the default of EOS_Omni::poly_gamma_ini to be whatever EOS_Omni::poly_gamma is. One way to achieve that would be to use some special value for EOS_Omni::poly_gamma_ini which would usually not being used to indicate that behavior and to make that default. However, this being an exponent, at least in theory, all real values are allowed. Another approach would be to introduce a new boolean parameter which would allow users to use an initial value for gamma which is different from the one of the initial data. This would be 'saver', but has the drawback of a new parameter. Nevertheless, I favor the second approach.