JohnS,
Not sure what you are meaning by the “universal mode” on the 798ci Si unit but it is not advisable that you use one of the DI transducers on any of the Si model units as this will put a strain on the 455kHz element.
No, a Universal Sonar transducer cannot be used on a DI model unit regardless of the set-up. There are some internal differences between the Universal Sonar transducers and our standard DualBeam transducers that can cause a failure in the DI units.