Powering 10 servomotors + Arduino
It seems like your power usage adds up. But ve aware that motors usually adds a lot of harmonics into supply lines, with that in mind some tips to keep everything running:


Decouple, decouple and decouple. Place a 100~200nF ceramic (smallest casing that you can) between VDD and GND as close as possible to every element on your circuit (arduino, motors, breadboard). Those will help clean fluctuations from the supply that could lead to erradic behaviour;
Don't shy on those power cables, they are carrying a lot of current. So use a wire gauge calculator for the amperage you gonna need and, if possible, go one tier up. That will prevent overheating and voltage drops along the lines;
Dc-DC converters are usually tunned for a specific current, so check the IC on the converters and the inductor they use. Go through the datasheet of the converter to check for efficiency with the current you gonna need. That is relevant because if the inductor/amperage efficiency table of the DC-DC converter points to, for example, to 80% 6V/5A gonna pull 3.125A from a 12V battery;
On the note above remember that batteries voltage drops along the charge state (12V li+ batteries run between 12.6V and 9V). So check if the DC-DC converter can operate in the full scale of the battery range. Check the power necessary on the worst case and adapt if necessary;
Place a large capacitor (can be electrolytic) close to the arduino decouple capacitor (mentioned in the first note), around 1~10uF/9V. That will work as a energy storage that can hold arduino's boot up current surge without stressing the circuit;
FUSE IT UP. short circuit in a battery is gonna turn your robot in a walking ball of flame. Rate a fast blow fuse or resettable one and connect it between battery and circuit.