will I need to run an external BEC if I hook it up to my Castle SCT/sidewinder esc?
Also what about the same servo with a Novak GTB?
trying to avoid using BEC's if possible.

You didn't specify which servo.

Regardless you do not need to use an external BEC in most cases but you may find that you may need a receiver capacitor if the servo pulls too much power from the ESC's internal BEC.

If you're using a Savox digital servo, there is a strong chance the capacitor won't be enough. I started there and it helped, but didn't cure all the problems I was having. In my opinion, get the BEC and save yourself the effort. This goes double if you are using Castle and Spektrum stuff together.

thanks, yes savox digital 1258