The box's suggested useage isn't really that big a deal.

For aircaft, you want response speed, which may not be as important for boats. Compare the reponse speed ratings with other servos.

I use a "Traxxis" servo that is labeled for RC car use as a rudder servo in some iarcraft. I wanted its power and the strength of its gears. Its response speed is the same as a "standard" servo, so I was not losing speed to gain power.

Generally if a servo is recommended as approptriate for a heli it fine for a fixed wing aircraft. The helis need faster response and more precision than the fixed wing models.

Inexpensive "Standard servos" will work fine in the U-Can-Do... however they will not give the precision needed for competition flying. (and the plane is capable of competition peformance)