You can only have 16 instruments in a song simultanously, but you can change the instrument on the part whenever yoy want. Each part has a MIDI-channel, and each track can send to any MIDI-channel. But each part can only play one instrument, but as I said, this can be changed using a Program Change MIDI-command. The rythm set can only be played on one part (normally 10). You can not use 2 rythm-sets on the same song. But I guess you can use program change here too.. But I haven't tried that.. Hope this was a little helpful. :-)

StigF