What is the "safe" amount of current I can apply to a DC motor?
You can only force one of current or voltage.  The load, which in this case is your motor, will determine the other.

For example, you can use a constant-current supply that drives 500 mA thru the motor.  You then have no control over the voltage.  When the motor is just starting, the voltage will be low.  As the motor speeds up, the voltage will go higher.

In your case it seems you want a supply that limits its output to 30 V or 500 mA, whichever is lower.  That is certainly doable.  It is only "forcing" one of current or voltage at a time.  When it's controlling the current to 500 mA, the voltage could be anywhere in the 0-30 V range.  When it's controlling the voltage to 30 V, the current could be anywhere in the 0-500 mA range.