Which DC-DC converter power supply to choose for a 6A motor from a 7.2V battery?
You don't need a separate DC/DC converter.  A motor controller that does Pulse Width Modulation (PWM) into an inductive (motor) load is essentially a buck-type non-isolated DC/DC converter.  At an intermediate duty cycle, the voltage across the motor will be less than the battery voltage - try measuring it.  Power is conserved, so 

Vbatt * Ibatt = Vmotor * Imotor + [switching losses]

Note that if you limit the maximum duty cycle, you can safely use a higher battery voltage than the motor can withstand.  This may allow you to add more energy storage (more batteries) to your project without being constrained by motor voltage.

Whether you decide to close the control loop with voltage (like a DC/DC converter), current (torque control), or go open loop (most small electronic speed controls) is up to you.