You're on the right track. The most straightforward approach is to compare the 6V servo ratings, they are available with enough performance for most from a variety of brands. Generally 200 or more oz-in of torque and 0.15 second for 60 degrees or quicker performance is considered adequate for a 1/8 by many. With metal gears.

If you feel the need for more performance, either need to add an external BEC at a higher voltage, or switch to an ESC that offers higher voltage BEC operation. Requires a high voltage servo in most cases (some regulars are known to work OK too). A receiver that can handle the higher voltage makes it easier, but is not absolutely required, at least for the external BEC approach.

Also be aware that some servos, notably at least some Savox models, may require an external BEC even at regular voltages.