For sanctioned combat here in the US, the scale planes must have been designed or in service between 1939 and 1955. I suppose you could make a ducted-fan P-80 or F-86, but it wouldn't be very competitive for the way we fly. Combat's all about quick acceleration and tight turns. We also use the propeller to cut the streamers, in addition to the wing.

If you're not flying in sanctioned contests, the rules are what you make them. You could have an all-jet class if you wanted.

Frankly, I don't know how well the powerplant would hold up. You never see scale turbine/jet/ducted-fan powered planes doing snaps and spins because the powerplants can't withstand the high-G loadings. They'll flex a little, and the impeller will hit the side of the duct. Ouch! I can't imagine the kind of internal damage that would be done in the inevitable midair collision. With standard propeller-based stuff, the prop absorbs most of the damage when it breaks, and engines are rarely damaged compared to the number of midairs.