If you compare the specifications of the two servos, you will see that they are for two completely different applications. The HS81 is much larger heavier, and more powerful than the HS55. It's obviously meant for larger airplanes.

There is no law saying you can't use HS81 servos in many park fliers, but when you get down into <16oz airplanes, an extra ounce makes a MAJOR difference on the way the plane flies, or if it will fly at all.

I recommend that you just get the Flash 5X transmitter only, then purchase the appropriate GWS flight pack for whatever GWS plane you choose. It will come with the tiny servos, receiver, speed control, and battery that the airplane needs, for less than it will cost you to gather the parts from separate sources.