Use 12v fans for your ESC/motor...

[BrianG]

True, the use of a switching regulator is definitely desirable in most cases, but the linear versions work fine for small loads below 100mA, especially for the price.

EDIT Oh, and I would never step up from a BEC's 6v to 12v. The BEC current would be twice that of the output current, not counting the efficiency factor. If your fan required 12v @ 0.2A, the BEC would be supplying a little over 0.4A @ 6v, which would take away from what the servo(s) might need. Always better to step down.