I can change clock to 100kHz when addressing 100kHz device.
But when I address to 400kHz devices on the same I2C bus, what will be the behaviour of 100kHz devices?
I am afraid , that they will behave unpredictably even though they are not addressed, but just because the clock is 400kHz.
Am I right?
That is the most likely scenario.
I would not change the bus rate. If I remember correctly, the I2C bus specification does not allow it. All the devices on the bus must run at the same rate.