The main issue is the voltage drop for the vin regulator for most Arduino's is 2V or more.
A 6xNiMh battery supply is a nominal 7.2V, but quickly drops below 7V, before it is even half exhausted. The 1300mAh rating might assume the voltage of each cell drops to under 1V, maybe as little at 0.9V, which is under 6V for 6 cells.
So those batteries will quickly drop to a point where the Arduino's voltage regulator doesn't have enough voltage headroom to operate reliably.
Worse, because they are low capacity, they are likely to have quite a high internal resistance, so when the motors are pulling current, the battery pack voltage will drop even sooner.
I think your best approach is the one commonly used by people making simple robots, use two battery supplies:
- Use a 9V supply (or 8.4V NiMh) PP3 for the Arduino barrel-jack or vin
- Use whatever makes sense for the motors, directly without a regulator
- 5xNiMh might be okay for servo's, or 4xNiMh may be better,
depending on the servo, again directly without a regulator.
- Join the ground (battery -) together, and connect to Arduino GND
You haven't told us what the specification of the motors are so this might be even simpler, and the motors might operate at the same voltage as the servo.
One approach for jetting multiple voltages for the servo and motors is to make your own battery pack. Make a connection at 6 cells, for the motors, and another connection at 4 or 5 cells (depending on the servo). This should avoid needing a regulator for the servo or motors, which take larger current.
If you can afford it, get something better than the L298D. It drops (wastes) a minimum of 1.8V at 1A, and a typical (more likely) drop of 2.55V at 1A. Worse 2A could be as bad as 5V, leaving only 2.2V to drive the motors! With something better, with a lower voltage drop, or low resistance, you could use one or two fewer NiMh cells, and still have the same voltage to drive the motors.