Hello,
Suppose I have added an SI-Drum Synth to my tracks. What I then would like to do is use some existing MIDI files to get ideas for a drum beat. So I look in the browser, open up the MIDI Groove Clips folder, select Drums. Then I could pick Electro and drag the Electro1_drm file on to my SI-Drum Kit track. It will sound good. But a lot of other Midi files are completely wrong. When I try to play Electro2_drm, I can only hear a tom playing and the rest of the notes are ignored!
 
Now I know you can change the key of the instrument and you can select different drum synths, but how on earth am I ever going to figure out what the correct key and instrument for all those midi files is?

WRT drums, the SI-Drum help shows the MIDI note mapping for the instruments in the kit. SI-Drums uses a subset of GM drums (extended with MIDI notes 22 and 26). You can change the keyboard in the Piano Roll View to show GM drum names by right-clicking on the keyboard, selecting the "Use these Note Names instead" radio button and select GM drums from the drop down. That will give you the full GM drum names instead of the keyboard. Even better learn how to use Drum Maps and clone the GM Drum Drum map and modify it for SI-Drums.
 
As for MIDI loops in general, if you have no idea what instrument the loop is intended to play start with something like TTS-1. It will play any note on the piano by default. It is also easy to change TTS-1 to play other instruments will previewing loops.