Separating the audio to different outs shouldn't change anything related to midi. But if you want to use a "blank" track that only sends midi notes and doesn't play audio, you can load an empty sample, mute/reduce track volume to 0, or indeed send to a separate out that is not in use.
Otherwise, Spark sends midi notes from each pad, I think you can change the notes that are sent via the option "choose midi drum map for pads" of the preferences panel. I don't think it is possible to send each pad to different midi channels, or to choose specific notes for each pad.