The authors have investigated the frequency plan of channels central wavelengths allotment for wavelength division multiplexing (WDM) systems with the mixed data rates and optical signal formats. Such mixed-WDM fiber-optic transmission system (FOTS) is offered as a model for the designing of a next generation of backbone optical networks. Since the necessity for transmission of different signal formats (even with mixed per channel bitrates) over single optical fiber may arise in the nearest future as a result of telecom networks infrastructure development effort which goal is a larger systems carrying capacity (data throughput). These frequency plans were drawn applying unequal frequency intervals which values were found using the principle of intermediate value applied for channel spacings defined in ITU – T Recommendation G.694.1. The obtained more accurate frequency grid of channel central wavelength allows increase the utilization of transmission band and spectral efficiency. It is found that the improvement in transmission spectral efficiency of investigated mixed WDM system highly depends on its initial configuration - i.e. equal channel spacing value that is used for channel separation and the number of channel with the higher per channel bitrate (i.e. 40 Gbit/s) comparing with bitrate in the remaining channels (i.e. 10 Gbit/s). In general, it is revealed that for some of investigated mixed-WDM systems the transmission spectral efficiency can be increased up to 42 percent by applying proposed flexed frequency grid for channel central wavelengths.