for pre-processing of the SAR data, calibration and Terrain Correction should be sufficient, maybe speckle filtering is also a good idea.
You can then create a stack of both data sets (can be found in the coregistration menu). If you use the supervised classification in SNAP you can choose Random Forest classifier. It can handle input data of different units.
If you want to classify in QGIS you need some kind of transformation to get both images scaled. Earlier versions of SNAP hat a scaling tool but I can’t find it in the newer ones. Have a look at image transformations in remote sensing.
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