Regulating a small clockwork synchronous AC motor
In theory, yes, it is possible to operate the motor at different frequencies if you keep the ratio of the voltage to frequency constant.  This is often called "V/Hz" or "scalar" operation.

In practice, at low frequencies, additional voltage is often added due to the effects of winding resistance.  This is called "voltage boost".

Operation at high frequencies is limited by the motor's insulation system's limits and/or mechanical considerations (e.g. balance of rotor/load, bearing alignment, increased windage losses, etc.).

The maximum speed for a motor (generator) is a standard design input.  As such, it can be exceeded only to the degree that the design has additional margin.  Since this margin adds cost to the machine/system, it is typically small.

Note also, that prolonged operation at higher speeds will reduce the life of the motor.

In general, provided the motor has sufficient cooling, there should be no lifetime issues with operating at lower speeds.  Cooling may be an issue in motors that were designed with a shaft mounted cooling fan and the assumption of operation at rated speed.  

In your case, your observed limit of ~2x speed on your motor is not unexpected and I suspect may be due to the windage loss torque reaching the the motor's peak torque output.  For low frequency operation, I don't expect cooling to be an issue since I'd be surprised if this motor had an integral cooling fan.