We have found that it is practical to run a small robot with motors like that and a Raspberry Pi off a bank of 6 AA batteries connected in series. The Understanding battery capacity- Ah is not A post on our blog might be helpful in understanding how to choose batteries for a robot. Generally, you will connect your various loads (the input to the Arduino board, the input to the regulator for the Raspberry Pi, and the input for the motor driver) in parallel to the batteries.
You will need to use a 5V voltage regulator like our D24V22F5 or D24V50F5 to provide a steady 5V to the Raspberry Pi. The current drawn by the Raspberry Pi can depend on the perepherals attached, so you should choose a regulator that can provide enough current for you configuration.
If you have not already purchased that RoMeo board, our A-Star 32U4 Robot Controller uses the same microcontroller (the ATmega32U4), a more modern motor driver, and it has a set of header pins that allow it to be placed on top of a Raspberry Pi.
Can you tell me more about the motors? What color is the code that is printed on them? That code does not indicate which gearbox is on them, but if you post a picture of them here, I might be able to help you identify them.