I use air unit caddx vista
The working voltage of this device is between 7 and 27 volts
I wanted to know the power supply for this device
Instead of connecting to the flight, if I connect directly to the 6s battery
It will not be a problem in the long run 

Yes, you can connect Caddx Vista direct to 6S LiPo voltage. If you do this, make sure you install a decent quality 35V low ESR Capacitor across the LiPo pads such as a Panasonic FR or FM series capacitor. If this is a 5" or bigger quad then use a 1000uF capacitor. For smaller quads use the largest value capacitor you can of 1000uF or below which will fit in the quad.