I guess it depends on what pylon racing and combat are like where you are.
At the clubs in my area they are two distinctly different types of airplanes.
Examples: pylon racers take off from the ground when the flag is waved (a holder holds the plane which is running at full throttle)
Combat planes have no landing gear and are hand launched.
Pylon racers are built for speed, combat planes are built for manuverability.
I suppose you could come up with something that can do both but it will not be the best for either.