Different digital signal input voltage than running voltage
No, this is not fine. Some inputs have protection diodes which are purposely integrated to prevent the thing you're about to do. In such event current flows from the input source to the Vcc of the circuit. If your input source has a low internal resistance, it's a straight short circuit, which most of the time triggers the overload protection of the voltage regulator in your circuit or destroys the diode inside the chip or far more.