To what extent are alternators "flexible" to variations in input speed and output voltage?
A wound-field alternator has maximum specifications, and can be operated anywhere within that envelope.

It has a maximum speed, dictated by the weight and build quality of the rotor.

It has a maximum field and output current, dictated by the maximum temperature rise. For a continuously rated machine, this is dependent on the cooling provided, but for intermittent operation, you can take thermal mass into account as well, for minutes or even 10s of minutes of very high current operation. 

There's also an absolute sensible maximum field current given by the saturation of the magnetic circuit. Whether the design makes that current coincident with the continuous case or the intermittent case is up to the specifier. The former will be more bomb-proof and less flexible, the latter is cheaper for the maximum intermittent power.

Within those limits, you can operate right down to zero volts output (field current x speed) and right down to zero torque loading (field current x rotor current).

For your battery charging specification, make sure you have an adequate design output voltage at your minimum motor test speed and maximum field current. For your motor load torque, make sure you have enough torque at maximum field and rotor currents.