You don't need the ballast resistors if you aren't going above 24v since your motors are rated at 24v, but you won't get high rpms. The current formula for volt optimum voltage is (SQ RT of Inductance) x 32. The old formula was 10 to 20 times the rated voltage.
Use a chopper like the one midway down the page here: http://pminmo.com/3axis/3axis.htm if you already have the driver, or design a current sensing circuit with your mosfets. There are many bipolar chopper circuits to learn from here on the zone. Why reinvent the wheel.