Which servos are you using?Not all hobby servos can handle 12v. Most of mine let the magic smoke out at that voltage.Many of the newer ones can handle it though.
What you have looks perfectly OK, with power for the Arduino coming via the USB. However, why not also connect your +12 volt supply into the Arduino power system and let the Arduino self-select between USB or main power.
Aren't all your servos 6V rather than 12V?
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